As long as your state is NFA friendly, you pay a $200 tax for machine guns, SBR, SBS, suppressors, destructive devices. "Any other weapon" (factory manufactured SBS, pen guns, etc) is a $5 tax.
You do have to file some forms with the ATF, as well as a couple of fingerprint cards and passport photos, and get your chief LEO to sign the forms. Then you wait for them to do a background check. These days, thats a backlog running about 6 months.
The "quote" button is at the top of the box you type in. Its the symbol next to the # sign on the right side.