A question about Amazon EC2/EBS Billing

Hey guys, Quick question. The EC2 pricing page says that Elastic block store is $0.10 per GB-month of provisioned storage. But I'm wondering what happens if I only use it for a few days? How would I be billed for that? As I understand it, every instance I launch will have an EBS to hold the OS, etc? So I'm wondering how much it will cost for each instance I launch? It seems expensive it they round up to am month of storage charges for just an hour's usage.