AWS is cost effective in some scenarios while not in others. It really strongly depends on actual needs, what resources are needed, in what proportion, etc. There is an article which tries to answer if Amazon is really the cheapest provider: http://blog.cloudorado.com/2011/08/is-amazon-cheapest-cloud-...

You can compare the prices with custom configurations of multiple providers with Cloudorado - http://www.cloudorado.com.