First, is it legal? Yes, so long as you follow the proper procedure and obtain Form 1 approval before you begin the build.
Second, how hard is it? Relatively speaking, not that hard. A suppressor must be properly built and has some demanding tolerances, but there are no moving parts.
But why? Because theATF eForm 1 approval process is 30 days while the paper form 4 (no e-filing available) is 10 months.
Is it hard? Not really. A good article is on Recoil magazine’s website here: https://www.recoilweb.com/diy-suppressor-155070.html
Is it expensive? No, it is probably less expensive than buying from a manufacturer.