Hi,
If you want to go with a prepaid model you would need to switch to Azure in Open subscription. With Azure in Open you can purchase minimum $100 credits from Microsoft reseller that must be used within 1 year after activation. Once you have used all of the credit balance your subscription would automatically be disabled unless you added a new credit.
Get Started with Azure in Open Licensing
https://azure.microsoft.com/en-us/offers/ms-azr-0111p/
There is no prepaid option with Pay-As-You-Go subscription type.
https://azure.microsoft.com/en-us/support/legal/offer-details/
Certain Visual Studio subscriptions include monthly "use it or lose it" Azure credits, however, considering your monthly spend I don't think paying the upfront purchase price would make sense unless you needed the subscription for other reasons.
If the above was helpful please click Accept Answer.
-TP