Is this the site? https://www.mooc-list.com
If so I can’t see why the costs are spiralling unless the site is receiving a lot of traffic - and even then it would have to be huge amount. If that’s not the case I would imagine it is down to the coding.
What are their current server specs/traffic?
With regards to Phoenix, many people porting from frameworks such as Rails have reported large cost reductions and something like 15x performance boosts.
Check out this thread on deployment regarding servers too: Deploying Elixir - opinions on Heroku, Digitalocean, Bluemix etc
