So what are the requirements / cost to become a class 3 SOT?
Becoming an SOT involves mailing a $500.00 check.
Nothing extra is done above and beyond your normal FFL background check.
Although I know curraheeguns is aware of this already, becoming just an FFL/SOT will not allow you to make a post 86 lightning link. You will need to be an 07 Manufacturer FFL and SOT. As an 01 FFL/SOT you could buy a post 86 lightning link, if you could get a demo letter from a law enforcement agency. If you think you're just going to go out and get your FFL/SOT and only buy/make some post 86 MG's for yourself good luck!