Cheapest Discourse hosting
NodeChef is both faster and cheaper than DIY. For a starting price of only $10 USD, you can have a managed forum up and running configured correctly with all the neccessary security features and performance enhancements.
Each additional 50k pageviews cost an extra $5/month.
Each additional 1GB of database storage cost an extra $5/month. You can enable database replication which doubles your database cost because of the extra standby server.
Each additional 5GB of file storage cost an extra $5/month