Total cost really depends on where you source the parts and how much you're willing to spend. It can vary all over the place.
To eliminate the hassle of retrofitting a 3rd member, I bought a complete (drum to drum) '99 axle w/e-locker for $550 and I'm planning to get an
ORS wiring harness for $160-$180. Total spent will be around $780. Still much cheaper to the comparable/alternative ARB locker.
criagslist.com is good place to find one, must be patient though, search daily, call and email questions. Most sellers who part out 3rd gens have no idea what an e-locker is. I was fortunate to find mine here through a local member.