"The cost difference is so large because this workload is not a very high EC2 utilization scenario. We need extra servers for multi AZ HA and to handle bursts."
Of course, this is all conjecture since I can't actually view your workloads, but we've been performing this exercise for our own services, and the t2 series of servers is remarkably capable: especially given their cost. The original administrators thought (or were lead to believe) that we really needed the raw horsepower that c*.large+ instances offer... and they didn't in most cases.
Routers, crud API wrappers, even some disk persistance applications all qualify as "non-CPU intensive".