(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.
It's not bulletproof, but it's surely better than the current case where long-abandoned works rot on floppy disks that can't legally be rescued. http://www.pcworld.com/article/248571/why_history_needs_soft...
If that's what they want, give it them. Bringing back copyright registration might not be such a bad idea, at least in a form where after 14 years of free copyright, you have to pay $1 per work per year and provide the contact name and address of the registered copyright holder. All unregistered works older than 14 years would automatically fall out of copyright into the public domain.