The cost varies based on the how much you can get used parts for.
I've never done the conversion, just read about it on the board. You'll need a front cross-member/suspension from a 2wd PU, a steering box, transmission & cross-member, new driveshaft and u-bolts to put the rear axle above the springs. You could do a custom 4 link suspension in the rear and get it lower but it will cost more. If you go 5 lug upfront, you'll probably need to swap the rear-end to 5 lug too. Trailing arm suspension from a PU can be added, but I guess there is a little fab work needed for the spring mounts or something.
There are several guys who have done the work.
Livrat's name pops in my head as the guy who has probably done the conversion the most.