Horizontal scalability is not always easy, but it is always expensive. Startups especially tend to discover the latter the hard way when they receive surprise bills from AWS.
You're right about price though; if the cost of compute is relevant in your business model - which, again, usually isn't the case - then doing that computation with Ruby on AWS isn't the best choice.
Surprise bills from AWS means it is time for refactoring ( and thus optimization time ;)