Agreed with AFJ.
The rebuild is pretty straightforward with the right tools. Do another search with some different terms and you'll find plenty of DIY 915 rebuild info. There's one topic that John Walker, Grady Clay, and a host of others walked a guy (rcecale) thru from dropping the engine to the finish. It's pretty good.
http://forums.pelicanparts.com/showthread.php?threadid=159059
I'm in the disassembly stages of my first 915 rebuild with a friend of mine. Doesn't look too daunting and we're a couple of knuckleheads. Most important thing is to have the tools. Mark's got synchro hub press tools, a press, a custom crows foot for one of the shaft nuts..........
You're right though. You can pick up a used one for the price of a rebuild. I saw one in the classifieds the other day for $1000. But you still end up with a used trans. in the end.