I always figured bought poisons fall into 3 basic categories, depending on your needs. And assuming you're the type to think of using them -- both from an ethical standpoint and a "do I really want to spend a bunch of money on a one shot item that's not a buff (ie, might not even work)?"
1.) No use. The poison simply isn't worth consideration.
2.) The more affordable poisons, which have very low DC and small effect (when possible, ABSOLUTELY sacrifice DC in favor of effect). They may do little and seldom work, but the point is, they DO still work, at least 5% of the time.

And if the poison's cheap enough relative to your wealth, it might be feasible to coat some porjectiles in it, spam them, and see if the investment pays off.
3.) When you
absolutely, positively have to kill something dead. Or you're about to face an encounter waaaaaay above what you can reasonably expect to survive. When you're just plain need to win, no matter the cost, it could be perfectly financially logical to go out and spend a major chunk of your total wealth on the highest freaking DC poison you can find. It's not an ideal situation, but one DC 20 debilitating poison at ECL 3 if successful will win the day like few other purchases could. Of course, this assumes you can acquire a poison whose price range is atypical for what a character of your level would normally be shopping for. Ie, you live in a big city.
Ironically the cheap spam poison is more of a high level option, and the super expensive 733+ poison is more of a low level option (because there simply aren't any good high level poisons, and even if there were, fort saves and poison immunities have skyrocketed by then).