At the moment, I have a database that isn't accessed very often, but has 400 million rows and totals out to be 100gb disk space. I'm worried that, despite the low usage, I'll have to shell out for a near enterprise-level database instance in order to get the storage I need. How does storage and database usage affect pricing? I've been looking at pricing on the AWS site for a few weeks now and honestly still don't know how to estimate how much this database will cost. Any help is appreciated.
are you looking at RDS or EC2 ?, I would suggest you go with an EC2 as you can get a EBS backed storage around 100GB for much less on a lower instance type.have look at this calculator it will give you a good estimate