How much would it cost to replace a final drive in a 94 Chevy Corsica 6cyl Automatic with Overdrive?

sounds like you've got the 4T60, or 4T60e (440T4) to replace the final drive, they've gotta pull everything out of the transmission - if the final drive went out, I can almost guarantee it went through the trans case, or at least cracked the trans case ... either way, you'd be better off finding a junkyard transmission.