Find a friend with a engine hoist, and go from there. I can't speak to how much it would cost since I've never paid anyone to swap an engine out...but I bet you'd save at least a grand doing it yourself-just in labor. Also, as mentioned you'd have to get a new k-member to fit the 5.0 into the '96, along with correct exhaust, trans crossmember parts, appropriate wiring would have to be carried over from the donor, and mated to the recipient vehicle, fuel system parts would have to be upgraded from V6 to 5.0 stuff, unless it would be either all stock or go carbed in which case the tank would have to be set up for a low pressure system.
If the '91 is a complete car, and you really want to do this, many of the parts should be able to carry over. I've never tried to use a Fox h-pipe or cat back in an SN95, but I'm pretty sure they're different, so you'd probably have to source one from a '94-95 5.0...same would go for the tranny x-member...the wiring would probably pose the biggest hurdle, but nothing that couldn't be done...just depends on how much effort you expend and how much you feel like spending.
I wish I could give you a good ballpark figure, but realistically, there are about 100 different ways you could go about it, and they all vary in cost. However, if I were to do this myself, and had to put a number down for an estimate I'd probably say about $1500. That would include expenditures for parts you know you have to have-as mentioned above and some of those would probably be used parts (k-member, trans x-member, exhaust, etc). But, the things you don't think about, like gaskets, plug wires, random sensors, clutch, etc will be found bad at the most inopportune time and slow your install.