I used to be a regular over at a Firebird forum years ago, and many people wanted to swap the transmissions on a 3rd Gen Firebird out for a T6 manual tranny due to reliabilty and whatnot. The average running price for a transmission swap was usually between $2000-3500 easy. And that was about 3-4 years ago. It's not a simple swap process, so you'll be shelling a lot out of pocket in labor costs, in addition to all the necessary parts and work to configure your vehicle properly. If you price out a new vehicle, you're looking at probably $1000 difference between auto and manual trannys (I haven't priced one new, so I'm just guessing.)
I think it'd be a lot cheaper and easier just to trade yours in for a manual (whether you do it now or down the road is up to you). But manual or auto, you're still cruising in a Mustang.