I used to run WP on $5 plan but the memory soon runs out (with the recommended swap config). The next tier seems to be working fine for the past few months.
You definitely need to be careful how to configure things, but enabling caching, event MPM + php-fpm, mod_pagespeed, etc. like we do in our Bitnami images helps a lot. The good thing is that it is relatively painless to resize if you need to.
