Flashbang is ... interesting. Instead of breaking ice it derezzes it at fairly great expense. Whether that's worth it depends a lot on whether the corp can rerez the ice quickly which in turn mostly depends on how expensive the ice is.

It costs 6 to derez a sentry plus 1 per strength (since you have to match the strength in order to derez, much like how Wyrm needs to match in order to reduce strength). So it costs a whopping 10 to derez a 4 strength sentry. 4 strength sentries cost on average 4.5, median 5 (here's the list). So you're paying roughly double in order to derez.

Here's a full calculation of the current sentries and cost to derez with Flashbang:

• Strength 0: 5 cards
• • Mean rez cost 1.6, median rez cost 1.00
• • Cost to derez 6
• Strength 1: 6 cards
• • Mean rez cost 3.3, median rez cost 3.50
• • Cost to derez 7
• Strength 2: 11 cards
• • Mean rez cost 4.1, median rez cost 4.00
• • Cost to derez 8
• Strength 3: 13 cards
• • Mean rez cost 3.4, median rez cost 4.00
• • Cost to derez 9
• Strength 4: 14 cards
• • Mean rez cost 4.5, median rez cost 5.00
• • Cost to derez 10
• Strength 5: 13 cards
• • Mean rez cost 5.8, median rez cost 6.00
• • Cost to derez 11
• Strength 6: 6 cards
• • Mean rez cost 6.2, median rez cost 6.50
• • Cost to derez 12
• Strength 7: 1 cards
• • Mean rez cost 9.0, median rez cost 9.00
• • Cost to derez 13
• Strength 8: 2 cards
• • Mean rez cost 15.0, median rez cost 15.00
• • Cost to derez 14

The 2-5 strength sentries are most common in the cardpool and there the rule of thumb that cost to derez with Flashbang is twice the cost to rez holds pretty well.

The value of Flashbang however depends strongly on how much better it is than other breakers. Mongoose seems to be the most popular killer, so let's take a look at how well it fares. The percentages are how much Mongoose costs to break all subs relative to Flashbang derezzing.

• Strength 0: 5 cards
• • Mean subs 1.40, median subs 1.0
• • Mean break cost 1.00 (17%), median break cost 1.0 (17%)
• Strength 1: 6 cards
• • Mean subs 1.83, median subs 2.0
• • Mean break cost 1.17 (17%), median break cost 1.0 (14%)
• Strength 2: 11 cards
• • Mean subs 1.64, median subs 1.0
• • Mean break cost 3.18 (40%), median break cost 3.0 (38%)
• Strength 3: 13 cards
• • Mean subs 1.69, median subs 2.0
• • Mean break cost 3.08 (34%), median break cost 3.0 (33%)
• Strength 4: 14 cards
• • Mean subs 1.64, median subs 1.5
• • Mean break cost 5.14 (51%), median break cost 5.0 (50%)
• Strength 5: 13 cards
• • Mean subs 1.92, median subs 2.0
• • Mean break cost 5.31 (48%), median break cost 5.0 (45%)
• Strength 6: 6 cards
• • Mean subs 2.17, median subs 2.0
• • Mean break cost 7.33 (61%), median break cost 7.0 (58%)
• Strength 7: 1 cards
• • Mean subs 1.00, median subs 1.0
• • Mean break cost 7.00 (54%), median break cost 7.0 (54%)
• Strength 8: 2 cards
• • Mean subs 3.50, median subs 3.5
• • Mean break cost 10.00 (71%), median break cost 10.0 (71%)

In the "big" 2-5 strength range Mongoose's cost to break all subs is about 40% of Flashbang's cost to derez, so derezzing with Flashbang costs 2.5 times as much as breaking with Mongoose.

What does this tell us?

In order for Flashbang to be roughly on par with Mongoose, assuming only one sentry per run (so Mongoose's restriction doesn't come into play) and only looking at the runner economy side, a sentry would need to remain unrezzed for 1-2 runs after the run in which you derez it.

But wait, there's also the cost to the corp that comes into play. To calculate the credit swing I'll assume that the corp rerezzes as soon as possible. In such a situation the derez cost of Flashbang is effectively a break cost, but we'll subtract the corp rez cost from the cost of the Flashbang to see how much it costs in terms of credit swing. Percentage is relative to cost of Mongoose to break (note: this is inverted from percentage in Mongoose table).

• Strength 0: 5 cards
• • Mean break swing 4.4 (440%), median break swing 5.00 (500%)
• Strength 1: 6 cards
• • Mean break swing 3.7 (314%), median break swing 3.50 (350%)
• Strength 2: 11 cards
• • Mean break swing 3.9 (123%), median break swing 4.00 (133%)
• Strength 3: 13 cards
• • Mean break swing 5.6 (182%), median break swing 5.00 (167%)
• Strength 4: 14 cards
• • Mean break swing 5.5 (107%), median break swing 5.00 (100%)
• Strength 5: 13 cards
• • Mean break swing 5.2 (97%), median break swing 5.00 (100%)
• Strength 6: 6 cards
• • Mean break swing 5.8 (80%), median break swing 5.50 (79%)
• Strength 7: 1 cards
• • Mean break swing 4.0 (57%), median break swing 4.00 (57%)
• Strength 8: 2 cards
• • Mean break swing -1.0 (-10%), median break swing -1.00 (-10%)

Flashbang is worse than Mongoose in terms of swing for most sentries, though it's not really bad.

There's a difference between you gaining (or not losing) credits and the corp losing credits. Sometimes it's better to spend some more money to make the corp spend more money or gain some other advantage (like running through ice for free). If that's your game Flashbang is certainly worth considering.

Inspired by Silv's analysis of Flashbang to Mongoose, I decided to try something similar comparing Flashbang to another Killer that has a derez ability: Golden. I'm going to analyse them somewhat in a vacuum, so ignoring abilities like Khan's and credits from Ice Analyzer.

Because both programs are 5 to install and take up 1, we only have to consider the cost to derez to compare them.

Golden requires that you break all subs, then spend 2 to return Golden to your grip to derez an ICE.

Therefore, taking n to be the number of subs and taking s to be the strength of ICE, the cost to derez an ICE with Golden is:

(2ceiling((s-1)/4) + 2ceiling(n/2) + 7)

Where the first term is the cost to boost, the second term is the cost to break, and the third is the cost to derez. In the case of Golden this is 2 + 5 to reinstall Golden. You also spend a to do this, but for the sake of simplicity I'll ignore this from the derez cost matrices below.

In the case of Flashbang, you don't need to break anything, so the cost to derez is:

(s + 6)

Making a matrix of ICE strength against the number of ICE subs gives the following values to derez ICE.

For Golden:
|---|-------------Strength--------------|
|---| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|Sub|---|---|---|---|---|---|---|---|---|
| 1 | 9 | 11| 11| 11| 11| 13| 13| 13| 13|
| 2 | 9 | 11| 11| 11| 11| 13| 13| 13| 13|
| 3 | 11| 13| 13| 13| 13| 15| 15| 15| 15|
| 4 | 11| 13| 13| 13| 13| 15| 15| 15| 15|
| 5 | 13| 15| 15| 15| 15| 17| 17| 17| 17|
|---|---|---|---|---|---|---|---|---|---|

For Flashbang:
|---|-------------Strength--------------|
|---| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|Sub|---|---|---|---|---|---|---|---|---|
| 1 | 7 | 8 | 9 | 10| 11| 12| 13| 14| 15|
| 2 | 7 | 8 | 9 | 10| 11| 12| 13| 14| 15|
| 3 | 7 | 8 | 9 | 10| 11| 12| 13| 14| 15|
| 4 | 7 | 8 | 9 | 10| 11| 12| 13| 14| 15|
| 5 | 7 | 8 | 9 | 10| 11| 12| 13| 14| 15|
|---|---|---|---|---|---|---|---|---|---|

Golden - Flashbang:
|---|-------------Strength--------------|
|---| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|Sub|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 2 | 1 | 0 | 1 | 0 | -1| -2|
| 2 | 2 | 3 | 2 | 1 | 0 | 1 | 0 | -1| -2|
| 3 | 4 | 5 | 4 | 3 | 2 | 3 | 2 | 1 | 0 |
| 4 | 4 | 5 | 4 | 3 | 2 | 3 | 2 | 1 | 0 |
| 5 | 6 | 7 | 6 | 5 | 4 | 5 | 4 | 3 | 2 |
|---|---|---|---|---|---|---|---|---|---|

So we see, at least for anything 7 strength and below, Flashbang is a more efficient card to derez ICE, as even when the cards come out equal, you've still got the cost with Golden. As strength increases, Golden begins to become more efficient because of the 2 for 4 strength boost ability.

Now this analysis assumes that you derez every ICE you encounter with Golden, which is certainly not the case. I can see Flashbang supported by Mongoose vs. Golden being an interesting deck building decision for derez focused decks. Do you opt for the derez efficiency of Flashbang or the flexibility of Golden?