COBRA costs you exactly how much you + your employer were paying for that insurance.

It's expensive because your employer's share of insurance was a significant part of your compensation (And because US healthcare costs are pants-on-head insane.) I'll point out that it's generally quite expensive to, like, stop getting paid.

Practically, post-ACA, health insurance in the US is about as tied to employment as having a roof over your head and food on your table is. If you don't have employment, or money, you're going to be in trouble - but that's the case with everything you need to live, not just healthcare.