Hey, it's the cost of credit.

With a prepaid plan, you credit the operator, because you pay upfront, and the service is rendered after it, and ceases if your balance goes below zero.

With regular plans, the operator credits you, and you can be late with your payment for many days before the operator ceases servicing you.

So it's a month worth if credit, plus a different risk profile.

Also, it's market segmentation: the prepaid plan is the gateway drug %)

There's no way the credit risk on the post paid plans are anywhere high enough to justify the higher prices.