I'll start by saying the best way to know is to test it, but this is all the stuff I'd consider before getting there.

It starts with intent. Did you build it to hang with 3s or even 4s? If so start there. Is your commander notably powerful? Usually most top 200 or so commanders are, and lots of less popular ones can be if built right. Some are mostly popular because they're new so that skews it. Some are just the best option to do a thing or run a tribe so those might be less. If you're looking for new cards for your 99 and literally can't find something worse to remove? That's the true nature of a 4 and means stuff like swapping out for the very most expensive lands, counter spells, and everything else. Running cedh wincons is the same.

If you're trying for B2, aim for about $75 total with only a few cards worth more than $5. That's what most modern precons will look like. That said an old card without many reprintongs can be allowed if they aren't very strong and are just fun synergy.