Radial to a 3/8x10 swap for a wood joint should not be much at all, I'd guess $40 for just a simple fix and maybe $60 if you are going from a wood to a metal insert.
Radial to 3/8ths will require a plug.
Bored, plugged, faced off, bored and tapped.
Going from wood to an insert still requires a plug.
The same except that you will be tapping for an insert.
Don't forget that when changing the threads in a shaft, you will also have to remove the pin in the butt. and essentially do the same steps on the butt as the shaft.
Add the price of an insert and pin to that. Radial pins are a bit more costly than others.
So you can more than double the time involved if you are planning on using the same butt.
I can't dispute what your repairman charges, but, lets just say that for $40, or $60, that job wouldn't be getting done in my shop.
Unless you brought beer. Oh, and pole dancers.