If the bad guys also had to pay $50/month/IP it would probably work.
The bad guys don't pay that much. And sometimes the bad guys actually use the IPs of other people (botnets on residential IPs) and don't pay anything at all.
They pay something. You can get a few ten cents per gigabyte for a voluntary proxy right now. I've never tried it long enough to get a minimum payout, so could be a scam for all I know (or maybe the minimum payout is the scam).
What would stop you offering someone a few tens of cents per GB to borrow any other token barrier you put up?
The bad guys don't pay that much. And sometimes the bad guys actually use the IPs of other people (botnets on residential IPs) and don't pay anything at all.