It is truly sad that bolt modding lowers the value of a Model M. I learned that the hard way.
After doing it a couple of times, and realizing how great it was, I thought that I could bolt-mod a few of the spares I had on hand, and make a few $20 bills in profit. Wrong.
Besides the hardware costing at least $10, shipped, etc, and the couple of hours work involved, it seems to me that a good bolt-modded M should sell in $100 range.
But, I suppose that it is foolish to expect to get more than the price of a new Unicomp.