1 bad publicity = 1 free credit for the runner to spend during each run. Therefore after 3 runs at 1 bad publicity the runner will have saved 3. The only way that math doesn't apply is if the runner doesn't use all the credits during a run. That generally means your ice is weak and you don't have anything interesting to trash (bad publicity credits can be used to trash assets during a run). Or, if you're overdoing it, you could have much bad publicity that the runner gets completely free runs through heavy ice and still have credits left. In any other case every run you effectively give the runner a credit.
3 runs isn't that much in the grand scheme of things so if score Illicit Sales it quickly becomes more of a money maker for the runner than for you.
On the other hand due to how bad publicity works the runner doesn't get that money into their savings account. That means they don't have it available if you decide to throw in a little Midseason Replacements.
Illicit Sales is good in a "get rich and kill the runner" deck, e.g. with GRNDL: Power Unleashed. However being a 3/1 it's not as good as Hostile Takeover since you can't score it out of hand. Also it needs to be scored with 2 bad pub already present to gain more credits than Hostile Takeover.
I don't see much of a case for Illicit Sales given the competition. The only way Illicit Sales would shine is in a deck which racks up massive amounts of bad publicity over a longer period of time, but I simply don't see that working given the economics.