I'll just say that the most I've ever paid for a Model M (not an M13 or anything made by Unicomp, mind you) is $4.99.
That might have something to do with the fact that I've gotten all of my Model Ms at Goodwill or Salvation Army.
But, that $25-50 estimate is probably good...
I did pass on a (Lexmark) Model M that I saw on eBay for $20, with $13 shipping, though... and it was Dell branded.