My November bill was about £58 quid for one machine instance running for the whole month, a couple of gig of data stored on S3, a gig or so of data in and out of EC2, a fixed IP address, and a fifty gig block storage device (aka drive).
Currency rate fluctuations apply when converting from USD to GBP - I think my bill has gone up about ten quid over the last few months
Of course one of the major advantages of EC2 is being able to scale both up and down on demand - if you get Slashdotted, for example, you'll have about 20,000 visitors in the first hour, gradually tailing off with a total of around 120,000 visitors over the first twenty-four hours - at least, that was what we saw when I
Slashdotted a friend's site the other year
An application doing a fair bit of work per request, as opposed to a static HTML page, would have trouble coping on just one server under that kind of load, but on EC2 you could just spin up a few more instances and load-balance across them, spinning them down as traffic tailed off - thus you only pay for what you need, even if you happen to need a hundred servers for just one hour.