The cost of creating an online marketplace mobile app depends on many factors, read more on www.codica.com/blog/how-much-does-it-cost-to-build-marketplace-website/, including the feature set you want to include and whether you're looking for a ready-made solution or to build something custom. Ready-made apps are often less expensive than custom apps, but they don't offer as much flexibility. If you need a lot of special functionality in your app (or if you don't want to wait around while someone builds it), then custom development might be the way to go.
I know you're going to be successful—you've got a great product and you're in an industry with a lot of competition. I think it's really important that you focus on making your app as easy-to-use, intuitive, and useful as possible. That's what will set you apart from the rest of the marketplace apps out there. Good luck!