The costs will depend on how much compute and storage you need. The prices are listed on the AWS website, so you can decide on what you want. I used cloudman to setup a multinode Galaxy cluster on AWS this summer for a training session at GCC2016 and was very impressed by how easy it was. Adding more nodes was trivial, the only thing that takes considerable time is increasing disk space, so don't plan to change that on demand.
For real advise on this, I would recommend either posting on the Galaxy biostar site or the galaxy-dev list, where there are a few people who likely host AWS instances.