Working With Rails

Non-developer here with a new start-up. I’m working on the spec for a web application that may or may not be a complex site. I believe it is on the complex end of the spectrum, but I may just be naive to the possibilities and capabilities of RoR programmers.

This site will move forward, although the company is in the process of funding “as we speak.” I know that a lump-sum, all-encompassing flat fee for a website/application isn’t practical.
Still, I need to come up with a general idea of the cost to get this site up. With limited technical expertise, trying to budget for web development is a black hole of confusion.

Could you help me understand the cost range I need to be thinking in with regards to having an application developed? And, how do I find someone to help me create a decent spec prior to development?

The site needs to act as an e-commerce site like Ebay, Amazon or Etsy with store subscriptions for clients (no auctions), a social networking platform, and a CMS.