If you do use spot instance with a local postgres on EBS, it comes down to about 4 dollars per month all together. With instance store, that should be around 3 dollars.
These are for t2 micro. If you use bigger instances, the money difference will be huge.
RDS is just way too expensive for small projects.
I do it partly to learn about aws features, partly because I would like if this plug-in existed myself and it doesn't :)