That is cheap compared to around here...
The "popular" axles like the 10 bolts the later GM trucks seem to lunch fairly quickly are the ones you'll pay the most for,probably due to lack of many "good used ones"...seen some salvage yards want $600 + for a good low mileage one..
Bigger rear diffs out of dually's seen to sell quite a bit cheaper for some reason..maybe more of them available,they don't go bad all that often,a friend paid $300 for one for a GMC 1 ton dually of 2006 vintage,its rotors ,calipers were junk, and housing were really rusted though,tubes had some flaking and pitting,but it came from a low mile wreck that sat in a field a few years..hard to find anything "old" here thats not halfway back to becoming iron ore again..