What would a reasonable cost be for a used transmission for a '07 Ford Fusion?

We have already given our mechanic $2000 up front for a used one.

You could get one rebuilt with a year warranty for about 2500 bucks with a nation wide warranty. ATRA.COM for more info

I think $2K is a pretty good price for a used tranny. What's important is that you get a warranty for it. Does that cost include cost of installation? Cost of labor? If so, then it's an even better deal.

You have to expect at least $2,000 for a modern car tranny. Sometimes, you think it'd have been better just to junk the car and get another one, so the $2K would be used on a better car. But $2K compared to $20K is always lots cheaper.