(1) It's low enough that it doesn't exclude anyone. If it was $10,000 per year or some other large amount, it would exclude a significant number of people.
(2) The aim of registration is not to extract fees. It's to make sure that works are registered; as soon as an organization goes out of business or loses track of what it's claiming, it forgets to register it, and it's public domain permanently.
Point (2) reveals a flaw, which is that organizations would spring up to do the registration (you'd pay them $100 and they would keep the registration going for 100 years). That's why the copyright holders' real name and address is required -- so that these intermediaries need to keep track of the claimant. You might also try banning such intermediaries, if that would work.
