I do some web programming, but I've got absolutely no idea whether I want to be involved in this. And there is no way in hell I'll sign an NDA. Honestly though, 50% is a lot for coming up with the idea. An idea is nothing unless you put the hours into developing it. As a freelancer, the people who just have the idea are some of the worst in my experience to work with. Not saying you are, but its a warning sign for me.
If your 50% includes the server, I'm sorry. But that should be in operating costs. Think of it as 20% profit to the developer (though, if you truly expect success, a flat sum paid for the work involved gives the programmer more confidence), 20% to you, and the rest into operating costs. Server, bandwidth, advertisements, support, oh shit fund, etc. If you end up with a large operating fund, you can either expand or have bonuses (to yourself or anyone operating with you).