The $500 minimum is for the 6-month and 12-month plans. Not for the "pay-as-you-go" plan.
The Pay-as-You-Go pricing option is extremely flexible. It involves no up-front costs and no long-term commitment. You pay only for the resources that you use each month.
Cloud Services and Virtual Machines are charged per hour of usage. Storage, SQL Database, Bandwidth, Caching, and Content Delivery Network (CDN) features are charged per gigabyte used. Some resources are charged per transaction.