There are a lot of extra costs in RDS that this calculation excludes: bandwidth, disk I/O, storage and backups. All that stuff is included in Heroku Postgres. Also, you're likely to get much better perf out of the $50 Heroku plan than the cheapest RDS plan because it's a slice of a bigger instance, rather than an underpowered VM.

So the real total cost comparison is a lot closer than it seems.