Adwords doesn't work like that. I think you must be confusing it with something else.
Anyway, here's a rule of thumb... ANALYZE IT
You can check in many instances whether its worth it to use adwords using simple calculus with contraints such as minimum cost per click (solver in excel can handle this type of analysis easily). You'd be surprised in how many instances that the optimal expenditure is zero, meaning you're likely to loose money with an adwords campaign.