This is exactly the sort of thing reserved instances would be good for (when you know it is going to be in use 24/7).

A three year reserved instance would be significantly less than $60 per month. The upfront reserve price of $300 over 36 months comes out to $8.33/month. Throw in an approximate cost of $14/month for the use of the compute time, EBS, network traffic.

That brings your monthly cost to just under $23/month, for a grand total of nearly $830 over a 36 month period.