I've picked up used 5.29 and 4.88 open 3rds for anywhere from $250-$350 depending on condition and vehicle. I made sure to find something that was ready to drop in, e.g. not needing to swap in new bearings or anything. Factory 4.88 3rds are great, but I don't think a locker is available for them (I could be wrong). The most desireable 3rd's are the high pinion fronts from Land Cruisers. They are great for lifted trucks because they reduce driveline angles. Note that these normally sell for a premium, sell fast, and can be hard to find.