I think I have about $300 into mine, but $100 of that was the reamer. I got my TRE's from NAPA, IIRC they were aroudn $130 for the set of 4.
You make good points about the ease of finding replacement parts. I carry some spares with me since no one else I run with uses them. If you get a spare make sure you get the one that goes on the passanger knucle. It needs to have the hole reamed so the DL will go in there. Plus since that is a fairly long TRE plus the hole in the middle, it has been known to bend.