The way they set the price on the stamp is they used the price of a Thompson. The government thought that doubling the price of a Thompson would discourage people from buying one.
Keep in mind, if you SBR the pistol, you will need ATF approval to take it out of state. If the brace laws were what they are now when I did my SBR, I would not have done it.
Fill out a form 1, send to ATF with $200.00 check, wait forever. When ATF decides you have suffered enough, they send you your stamp. Have rifle engraved, install stock, have fun.
I have a HK51 with a 7.5 inch barrel and it runs perfect on milsurp ammo. I ran some rounds through the chronograph with the 51 and my Remington 700 with a 26 inch barrel, the rounds left the 700 at 2650 fps and left the 51 at 1900 fps for the same load.