"A gunsmith should be able to machine an M-rod barrel for around $50.00 ."
Appears to me that you place very little value on the time and skills of a gunsmith/machinist spending an hour or two to machine, and fit an extended barrel to a Mark II.
However, you seem to have a very over-rated opinion of the value of your own time:
"I charge an extra $50 on top of actual shipping cost to send out of country."
I'd venture to say that Walter's estimate is far more realistic.