Cloud-based Database scaling, security and availability

We have an mobile web (ASP.NET + MS-SQL 2008) website that stores users' "personal" data. We are n开发者_JAVA技巧ow developing an iPhone & Android apps that would sync mobile data periodically to the same backend. What is the best way of going about it for hosting in the cloud? My requirements in no particular order:

• Speed/Performance
• Security (we bcrypt the data etc.), but the database needs to be secure, since a hint of database being "hacked" can affect perception
• Availability: need HA
• Finally $ - want to maintain costs low.

Some I have looked at are: Google data-store (problem: no RESTful API), Amazon SimpleDB (the cost seems quite prohibitive, has a WTF small 10GB "limit").

Any recommendations on how best to architect this solution?

We decided to go with Amazon RDS https://aws.amazon.com/rds/. It is still in beta and seems price reasonable. Hope this helps someone.