What % to offer bill collectors in negotiations?

My daughter owes $3000 for hospital bills, now with bill collectors. I offered to pay 1/3 to the hospital, but they turned me down and sold the debt to collectors instead. How much does a hospital get on the dollar in selling such debt? How much would the collector settle for? (My daugher is unemployed and without ets. I want to help, but not with the entire bill.)