If you are going to host it for a long duration, you can reserve a heavy utilization micro instance and end up with an instance cost of less than $6.50 per month (with a three year reserved instance).
As long as you aren't serving lots of images, this will keep you well below $20/month - even with large amounts of traffic.
If you are serving lots of images, or worried about excessive bandwidth: Linode gives you 20GB/200GB (storage/bandwidth) on a 512MB server for $20, and MediaTemple gives you a 512MB server with 20GB/350GB for $30 (a little cheaper if you are going to be in the 300GB range when you consider Linodes .10/GB for overages).
As long as you aren't serving lots of images, this will keep you well below $20/month - even with large amounts of traffic.
If you are serving lots of images, or worried about excessive bandwidth: Linode gives you 20GB/200GB (storage/bandwidth) on a 512MB server for $20, and MediaTemple gives you a 512MB server with 20GB/350GB for $30 (a little cheaper if you are going to be in the 300GB range when you consider Linodes .10/GB for overages).