I would obviously pay them if/once the idea took off, but I don't have the capital to do so now.
Listen, I get phone calls and e-mails like this at least once or twice per day. I want to embrace the caller, but honestly, these types of calls and questions, while well intentioned I'm sure, can take up huge amounts of time and effort (and as you indicate) pay nothing and statistically most startups and new businesses flop within their first five years.
You need to complete a business plan. If you can't complete a business plan because you don't know what to add or put into the spaces, you need to read (at least) several books on how businesses start and function. I would read at least two or three business books by people who have started businesses.
Businesses need to have multiple ways to generate revenue or they fail and take homes and cars and families with them. Having an "idea" is not a business. You need to have multiple ways to promote your "idea" into a business and find ways to find out if it's even realistic.
It's unfair for you (and some would even say unethical) to expect a developer/programmer/coder to work for you for free, spend who knows how many hours picking your brain trying to understand your concept, figure out how it would make money, who other competitors are (because I can assure you this "idea" already exists and is being executed as I type this sipping my morning Earl Grey) and then how to responsibly execute the idea as an app or site, while getting nothing but a smile or handshake in return. And even if you can get someone to do this for free - the results will be in question, because who's going to do their best work for you for free?
You have a responsibility as a thinking, feeling, cognizant biped to bust open a few business books, finish a business plan if you have not already done so, meet with a few local business owners who are doing similar work, and figure out how this idea would generate real profit you can pay bills with before speaking with a developer. Don't worry about someone taking your idea; because it's a) unstructured, b) not making any money, c) is already being done. You may have a unique spin on some kind of idea or practice but you should go to www.score.org, read a few sample business plans, read some related articles and do alot more studying and preparation before engaging a professional to help you build a castle in the air.
Sorry if I'm a bit brusque-sounding in this response, but from the business owner's perspective, your question puts the workload on someone else where it's clear so much more discovery needs to be done on your part, and I personally don't encourage people who aren't developers contacting web developers/programmers/coders asking for free help defining a concept.
You'd never call a local dentist and ask him or her to look at a tooth for free. You wouldn't expect a chef to prepare a meal for you for free. You wouldn't ask a mechanic to repair a car for free because you have a dream of driving cross-country. So, it's unfair (and unrealistic) to expect a coder/programmer/developer to spend untold hours on an idea for free.