I think I understand your situation. AWS costing can vary depending upon the various AWS tools you use to assist your solution. External volumes, Elastic IPs to mention a few.
As per my understanding, you are looking for a basic VOD solution with not much load. So you can probably give the 'Large' instance a try and see what it costs you over a period of days.
The costing model is on a per hour basis. So you can do your calculations accordingly.
Once you decide to scale up, you will need the Larger instances or probably even a combination of them to get going. But I suggest you at least get a hands on with the least costly instance first and then decide.
Let me know if you have any other concerns.