I'm not sure which region you priced, but an m4.16xlarge in us-east-1 (Virginia) is $3.20/hour, or $2304/month.
But if you are comparing it to a purchased server, you ought to be looking at Reserve pricing, a 1 year reserved instance is $16K/year, 3 years is $32K (or you can pay monthly if you want).
Still not as cheap as owning the server, but not quite as bad.
With an m4.16xl, there's no effective difference between a dedicated or default tenancy instance - only one .16xl can run on a physical hypervisor, which is reflected in the pricing, it's $3.20/hour whether it's dedicated or not:
For smaller instances, there is around a 10% increase for dedicated, for example, for an m4.4xl, the price goes to $0.88 for dedicated tenancy versus $0.80 for default.
There is a $2/hour fee for each region in which you have dedicated instances, but that gets lost in the noise if you're using significant AWS resources. (it's $2/hour regardless of how many instances you're running)
We haven't noticed any significant difference between performance on dedicated and non-dedicated instances, and only use dedicated instances where required for compliance/regulatory reasons.
But if you are comparing it to a purchased server, you ought to be looking at Reserve pricing, a 1 year reserved instance is $16K/year, 3 years is $32K (or you can pay monthly if you want).
Still not as cheap as owning the server, but not quite as bad.