Why one transfer of any amount within a 30 day period? It seems like it would be so much simpler to make the minimum outbound transfer $25 OR the full balance of your account if it's less than $25. It seems like they really over thought the solution here.
ACH transfers aren't free--Prosper has to pay a small fee for each ACH transaction. The minimum transfer amount was to minimize frivolous ACH transfers. By changing to a $25 minimum or 1/mo any amount, they are still discouraging frivolous ACH transfers, but still allowing those with a valid need to withdraw small amounts to do so.
Overall, I think this is a reasonable approach.