All CloudVPS plans include a SQL database from our SQL cloud, which is billed based on how much you consume ('consume' meaning the the average amount of SQL disk space you're using for the given billing period). You get your first 20 MBs at no charge; each additional 10 MBs of disk usage each month will be billed at $0.50.
You can also add SQL databases to your account via the WebControlCenter: Menu > Database > MS SQL 2008 > MS SQL Admin.
Additional databases are also billed based on consumed storage, at the same $0.50 per 10 MB price.
So, for example, if you average 100 MBs of SQL storage per month, your monthly cost for this would be $4.00. This would be the price regardless if you had 1 SQL database using 100 MBs or 5 SQL databases using a combined 100 MBs.