That's where EC2 starts to make sense - not always, but often.
For your site, without knowing the details, I have no doubt that you made a sound decision. You should just be open to revisit that decision once it grows to a point where a few hundred bucks difference a month don't weigh as much anymore. Or rather, where the added flexibility makes that difference worthwhile. - That's all I was trying to say.
You also do not differentiate between traffic in and out ("in" is currently free fo EC2 and in any case cheaper than traffic out) - this may be significant depending on the kind of service you run (for most applications it won't be).
Similarly: if you make use of other Amazon Services (in particular S3 which you too seemed to like) this would count as external traffic from servers external to amazon but not from EC2. However, again, for most applications that would probably not change the overall picture.
But i do see one argument that can change the picture: if you use ec2 for overflow, testing and short time workloads and some other hosting (in-house?) for everything else, then this significantly complicates your infrastructure, i.e. you need take care of your in house servers and understand ec2. This may be significant enough to run even long running servers in the cloud.
I wouldnt say text sites pushing over a TB are rare. I have a few myself doing over that ( petitionspot.com does 2+ TB a month ) - That particular site runs on a 5520 QC with 6 gigs of ram and 10tb transfer for 200$ a month )
If I was to put that same site on EC2, I would be paying significantly more, as I doubt even the large instance could handle it. 1.2ghz opterons are not exactly speed demons.
For the sake of argument, lets say the Large Instance could handle it though.
Pricing at 34 cents a hour = 252.96 a month Bandwidth costs, 2TB = 340.00 a month
So we can either pay 592.96 for a less powerful machine, Or 200, for one that has 8TB more bandwidth to play with, and has roughly twice the processing power. In case you are feeling frisky, you can upgrade the ram to 12gb for 75$ a month. Still significantly cheaper.
https://orders.gigenet.com/index.php?order=true&form=235... Heres a link to the server I mention above.