That may or may not be a wise move depending on the idea itself, and, of course, the agency.
Some development houses specialize on building products up from the idea. Some others are much better at taking a ready product and improving it. It's largely an issue of process and methodology.
I'd personally second what Brandon suggested. Get a CTO or someone to work as a product owner on your side. Then do one of the two things: get a high-end shop that specializes in mobile product strategy and have a basic prototype made. Why am I emphasizing the point with mobile product strategy? Because that indicates they mean business. They dive deep. They're most likely to get the job done.
OR make basic design/specs in-house and take them to an agency. That works pretty well, too.
The workshop option is another good way. We've got the same practice at our development agency
. For a very reasonable price, you'll get all your questions answered, and then make the choice whether you're comfortable to move on with that particular shop, or move on to someone else.
Feel free to contact me in case you need a consultation.
Oh yes, you know how much are ideas worth. Zero. It's all about execution and people.