80% of my costs come from securing client data and having a HA environment. We were using Oracle DB for many years, switched to PostgreSQL last month due to usability problems, not cost.

I hope you make clear to your users that there will be data lost (to the point of your last backup) in an event of failure (which is certain thing in AWS).

If you think Multi-AZ will save the day: we lost a whole region of data in 2011 and again in 2012. If we didn't have a slave node on another region, we would have to restore a 13 hours old backup, that would pretty much make us go bankrupt.