Perhaps you could have a ratcheting cost, where the first 100 domains are $10 each, the next 100 are $20 each, and so on up to some upper bound, with provisions

Perhaps you could have a ratcheting cost, where the first 100 domains are $10 each, the next 100 are $20 each, and so on up to some upper bound, with provisions for shell companies that solely register domains.