Usually when a synchro goes bad, it takes one or more blocker rings with it. A full set of blocker rings and new 3rd gear syncho (why is it always third??!!) runs $336 shipped from Bill J.
The labor, on the other hand, is something else. You have to break the trans case open (duh) and remove both the main and countershaft. You'll need a new bearing and race (the bearing almost never comes off "gracefully"), and the worse case is you have to cut it off with a die grinder. It can be a fairly easy job, it can be a b*tch and a half... Depends on the bearing needed to get the gearset off the main shaft to the 3rd gear synchro.