How much does it usually cost to host a small Wordpress blog on Amazon?
First off: S3 is only for storing static (unchanging) content like images or text files. Wordpress doesn't fit this setup - it rerenders the page every time you view it, so you really want to be comparing your current MediaTemple setup to Amazon EC2, which lets you rent a computer by the hour.
The smallest instance size is a Micro, which provides 613MB RAM - more than enough to run a small Wordpress installation.
Instances are $0.02/hour, or about $15/month assuming you want your blog to be up and running 24/7. Micro instances come with no disk space and need an EBS volume to have a place to store your data ($0.10/GB/month) and bandwidth is extra, so assume all together somewhere just south of $20/month.
You can get cheaper if you're willing to prepay for a reserved instance - for example, $82 up front lets you pay $0.007/hour for the instance, which over 3 years lowers the instance hour cost to under $10/month. You'd still need disk + bandwith, so the absolute cheapest this way would be about $15/month.
Remember, however, with EC2 all you get is the raw resources. With great power comes great responsibility. There's no hand-holding on getting things running, maintaining security updates, dealing with machine crashes etc. That's probably a large part of what a company like MediaTemple is providing, and by switching to EC2 for a few buck less you'll be losing that.
They also have a free offering for one year: http://aws.amazon.com/free/