If you have bills sent to collection do they penalize you by number of bills unpaid or amount or both?

I have many medical bills unpaid, (over and above my 90% my health care coverage). I feel they are billing me whatever they want and have let several go to collection. I’m wondering how badly this will hurt my credit and the best way to deal with it. Once they’ve gone to collection is there any reason to pay them? I’d love advice about how to handle this.