How much are you really wanting to spend? Also what year/make/model/seat style of the vehicle. I used to work in a interior shop so i have connections I just hated half the new ones cause they glued the material to the foam.
Are you looking to stick with a complete leather package, or go for cloth? Completely replace the covers or slide over? Leather definitely can't be had for a few hundred bucks....at least any of the good ones.
The last leather kit we installed which was in a Chevy Suburban like my bro's, which is 4 bucket seats, a rear bench, was around $2,000 or more, it was a pretty costly job that was a pain in the neck as well, and he would estimate through his supplier to be around 700-1000+ depending on how high of quality you wanted. If you wanted, he could call and see but he just had time for a estimate when i talked to him.