You should not have to pay for another registration fee once a space opens up....you've paid that fee and that fee allows you to be on the waitlist.
I had a friend tell me that she paid her money and waited on the waitlist and once a spot opened up in the spring she was let in easily because she already filled out the forms and paid the fees - she did not pay another fee.
Do you have a copy of the forms you signed? If not, I would demand to see them to see if the "small print" says anything about conditions for refunds.
If you want to remain on the waitlist, unfortunately they can keep that money, but if you no longer want to be on the waitlist you should get some of it back. And you should NOT have to pay another fee if a space opens up. I mean, your child is not technically "registered" until he/she is accepted/attending school. How can they charge a "registration fee" when s/he's just waiting to be in school? They should call it an application fee or processing fee -- if they are saying it costs something to file a piece of paper on a waitlist.