While I think $1250/year for a /24 of space seems a bit high, I see no reason that legacy allocations should remain free. Perhaps $100/year (like an ASN is) would be reasonable for small legacy allocations. This is especially important for end users who have these allocations as they would most likely be free from their upstream provider.
That being said, if it is larger than a few /24s I see no reason to not have the regular rates apply. If you have a /16 and can't afford the fee, you can't possibly afford to fill it with machines and should simply be allowed to swap down to a smaller allocation. Such a scheme would be in the best interest of all as it would all for some reclamation of numbering resources.
Charging something also seems as though it would help with the IP hording problem that is going on with legacy allocations. It would also help to "automatically" expire allocations which are not in use as users would be less willing to pay for resources they are not using.