Remember with Rackspace Cloud you are also charged for "compute cycles" (a measure of CPU, memory, and database operations), storage space and outgoing bandwidth. They know that every incoming GB of data will have to get processed on the server, so they'll get you with the compute cycles, and storage, and you're going to have to send a response back to the client, so they'll get you with the outgoing bandwidth.

I host with Rackspace Cloud, and I can tell you that compute cycles go by pretty quickly with PHP/MySQL operations if you're not careful.