The Matrix isn't as hot as they'd have you believe. Right now the Camry is their bread and butter, and the Yaris is their new kid on the block. MSRP of the base Matrix online says $15,410. I figure you're probably going to have to shell out a few grand for the necessary modifications, so start at $13,000 and work from there.
Do you mind if I ask what type of dealership you're buying from (big city, small town, downtown, suburbs, etc.)?
I wish you the best of luck. If there's anything I can do, don't hesitate to ask.