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 25 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" 25 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 12 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.