I've experienced that same sticker shock. I'm assuming you've looked at the tenon cutters from Lumberjack Tools or Veritas, like in the attached images.
We have a lot of standing dead, perfectly straight aspens on our property that I've been clearing for fire mitigation; I've been saving the nicer logs for use in furniture or perhaps various trim pieces for the house. I wish that were the only job on my house project list!
Gary Hart built a log tenon cutter from some industrial plastic, here: Homemade Log Tenon Cutter .