If you're happy with your concept, you've road-tested the business plan by engaging prospective paying customers and convinced you have the specification nailed, minimising cost may not be your best focus.
If you do want to do cost minimisation, if you're technical I'd +1 for http://www.odesk.com - for me, I like their way of creating a virtual team, whether your associates are half a city or half a world away.
My practice is generally to get a site up to test my idea. That's not always the famous 'minimum viable product,' but I definitely expect there will be a lot of change between what I know is a great idea and what customers agree is a great idea.
So if it were me, I'd probably use the learning so far (on the proposition and on the build costs) to pare back.
However, if I had already done the work and knew what I had was already as lean as I could live with, I might look to negotiate.
An interesting follow-on conversation with the design firms you've already reached out to, who've convinced you they're competent to do the job, and who seem actually excited to get involved could run like this:
"I love your proposal. And I'd love to find a way to work with you. So here's my ask. You've seen what I want. That's where I'm going. But I won't commit $25k to my first step. I've set aside $10k in the budget, plus the time I'm going to commit building the relationships with the retailers that will bring the site to life.
"So, could you give me a new proposal for $30k?
"Step 1 will cost $10k. It will maximise my go-to-market and really show off your talents. It will capture all the data I need, even though some of it won't get used to start with. It will make sense for its users. Step 1 will bring my vision to life, and will get us working together.
"And after that there will be two more steps. That will get me nearer to, and then right up to the whole vision.
"I'm looking for a partner. But I'm not asking anyone to share my risk. Would you be able to work like that?"