I would obviously pay them if/once the idea took off, but I don't have the capital to do so now.
While I appreciate your reasoning about not giving away your idea, you have to look at it from the developer point of view too. If a developer works for you, he has to do it for free with a future uncertain payoff, especially as the idea is unproven and has no guarantee of success. The only way someone would work in this situation is if s/he has some assurance of a decent return on the risk s/he is taking on your idea. That would mean some part of ownership in the business. Also, you will find it hard for someone to do this full time without any additional income to support him/her. Which means you are looking at someone doing this in their spare time.
Also, remember, there are tons of ideas out there, what is important is how well you execute on them.