[Feature] Check instance pricing for On-Demand

We were using the cpus and memory arguments to match our computing needs.
However, we noticed that mapt regularly launched expensive, workload-specific instance types (GPU accelerated, network optimised, large local storage, etc).
While these instances indeed had the specified cpu/memory configuration, there are plenty of cheaper, more generic alternatives and we don't require the extra technologies the expensive ones provide.
Example: both d3en.12xlarge and m5a.12xlarge have the same vCPU/Memory amount but the former is about 3 times more expensive.

As a workaround, we're now using the compute-sizes argument, but it would be great not having to hardcode specific instance types.

Would it be possible for mapt to do a cost comparison before selecting an On-Demand instance type, same as it does with Spot instances?

Start by locating the existing Spot instance pricing logic and the On-Demand instance selection path. Compare how pricing should be applied when choosing among instances with matching CPU and memory, and verify the result against the d3en.12xlarge and m5a.12xlarge example without requiring hardcoded compute sizes.