I would obviously pay them if/once the idea took off, but I don't have the capital to do so now.
First, consider that your idea might have already been executed and you just haven't heard of it. I've been to startup events where almost half of the ideas presented were already executed years ago--the founders are always shocked and their faces fall in disappointment. This is so common because very few people have a photographic memory bank of all the obscure startups that ever existed! This is probably not the answer you wanted ;-)
Even if the idea hasn't been executed yet, don't kid yourself--there's 100s of other developers out there w/ your same idea somewhere in their todo list. Lots of us were building social networks at the same time as Mark Zuckerberg. Ideas are a dime a dozen. Hopefully you realize most developers have pet projects of their own that they would rather work on. Developers throw great software ideas in the trash all the time because they work with software every day. Developers are short on time and money, not ideas.
In short, I think you're overestimating the appeal of your idea. Maybe a good business idea would be to ask developers to unload their stash of "pretty good" ideas they don't have time for, then charge a monthly subscription and pay developers a percentage of the revenue based on how well their ideas are rated/ranked. I used to offer a "free idea of the day" on my blog but nobody seemed to care--go figure! One of my free ideas (years before it was announced) became the Kindle and Amazon still hasn't added all the features I came up with in 2004.