Buying USED is always going to be cheaper than building. What all are you wanting for specs? Surely you could find one already fully built locally for less than $3k like greatwhitez71. For that kind of money you could buy a used one already geared, locked, trussed, and full chromo shafts with CTMs or Superjoints. Heck for $3k you could probably buy a really nice SET of fully built axles.
Going rate for a good condition SET of chevy tons in my area with 4.56 gears or deeper, lockers, etc is $1,500-$2,000 with the normal goodies like diff covers, pinion guard, perches or link brackets, etc.