Is there any other provider which is cheaper than Rackspace? I know they provide an hourly plan, but I would like to stick with a monthly plan.
That's a tough one as these are expensive to setup with a very large investment by the host. I know Cartika offers their "cloud" hosting which allows multiple domains etcetera, but they are priced at the same monthly cost as RackSpace/Mosso cloud sites. I doubt you will find a "low cost/budget" host providing a true load balanced scalable cloud type environment.
It doesn't really get any cheaper than the Rackspace Cloud Servers platform which starts at $0.015 per hour. Remember, the hourly billing is calculated throughout the month and charged at the end of each month in one lump sum. So if you use your Cloud Server(s) continuously it is just like being on a 'monthly' plan. But if you do delete a Cloud Server instance which you don't need anymore you get the benefit of stopping charges for that server instantly.