Hosting with Amazon advice

Hosting with Amazon advice

I was looking into .NET hosting and came across the Amazon Windows
Hosting. I'm looking for a solution with IIS, ASP.NET & MSSQL.
The website will be accessed mostly for 2 particular months in a year so
there's no need to stay with the same specs for the whole year - I still
need it to be online for the whole year. For this I was thinking to
upgrade/downgrade accordingly from micro to small/medium when needed.
Guess I only need an EC2 instance only with everything on it? Or should I
opt for an RDS too?
Also, something else that I wanted to be sure of. When Amazon say that you
have to pay $0.09/hour, if for example:
Server is used 5 minutes between 07:00-08:00
Server is used 25 minutes between 08:00-09:00
Does that costs $0.18cents (2hrs) or just $0.0045cents (30 minutes)?