Well like you said, any ballpark quote without knowing anything about the project would be meaningless to you. Also, you may want to look for a designer along with a developer, since a prototype is mostly about the user interface and not so much about the website actually working. To put together a flashy prototype I would start with a designer who has an online portfolio that wows you.
Then you can bring in a developer to get a few of the key pieces working-- or at least make them look like they work. It sounds like your primary objective at this point is to score funding, which requires the ultimate sales pitch (elevator pitch if you prefer), which starts with an amazing prototype, which if it were me I would want a designer with a good reputation to deliver.