Amazon has finally announced a database type web service, SimpleDB, which will be going into Limited BETA in the next few weeks. this will allow you to store, query and process data in a structured way which will be stored in the cloud. pricing is as follows:
you are charged 14c per hour of machine utilization, which is calculated based on Query, Get and put requests. Next, you are charged based on data transfer (10c per gig in, 18c per gig out for the first 10tb, 16c for the next 40 and 13c per gig over 50tb). storage is charged at 1.50 per gig per month. there is a lot of info about this on their main page.
I have requested access on their site, and if I get access, I will do some testing, etc. Could be very sweet!