You'd probably shave much more that $400 by using dedicated servers (rented monthly) rather than EC2 instances.
Example pricing (from OVH):
http://www.soyoustart.com/en/essential-servers/
(You'd probably need to do some homework to find the most price effective platform for your workload. For CPU intensive workload, I usually start with https://www.cpubenchmark.net/ given than OVH gives the exact reference of the CPU they're using).