[ddubois]

As mentioned, there are a number of factors. It's not just pot odds, but also: "Will this opponent give me his whole stack if my draw comes in?"

It's probably possible to make a mathematical formula that emcompasses the size of the pot, the odds involved, and the stacks size of both players. I think it would be complicated though.

Let's look at one example:
Stacks: Hero 700, villan 900
Blinds: 15/30
Pre-flop: Hero bets 100, one caller form the BB
Flop: Pot 215, hero with TPTK bets pot 215. At this point it costs villan 215 to get a shot at somewhere at least 430 and maybe as much as 815. Paying this fee only lets him see one card, and he will catch his draw roughly 1/4th of the time (again, varies on number of outs). So far, it looks like villan would be making a small mistake to call.
Turn: Draw missed, pot is 645, hero has 385 left. Hero pushes, now villian is getting charged 385 for a chance at 1030. With only a 1:4 chance of hitting, again, he's making a mistake to call.

The sucky part about this is hero has to commit his whole stack if he wants to charge the draws enough. So if he's behind a set or two-pair, he's screwed - there's no way to bet small enough to get away without opening a hole for flush draws to slip through. If someone has more than 9 outs, like an open-ended straight-flush draw, you are definately SOL. There's no way you can charge them enough, at least not on the flop. (On the turn it might be possible, depending how much stacks are left.)

Change hero's stack to 2000, and and two things change: 1) villan's implied odds on the flop change to a range of 430-1015, which means 215 might not be charging enough if hero will pay off, and 2) it's much less painful for hero to bet the requisite amounts to force out draws, without worry of being eliminated by 2-pair/sets.