It's already possible. Set the price on a biller at $1 a day. and let the member buy the number of days he wants.
Then tell affiliates.
In the real world, memberships cost the sponsors $10-15 a month, approx. The other $15-20 goes in marketing and profit. Affiliates simply will not send traffic to a micro payments site in the numbers required.
The $30 a month price is set for a reason and it has nothing to do with the product.