Also, as a benefit of rehabbing, a lot of those penalties will fall off. 

For example, I had $21k in student loans before default. That skyrocketed to $28k. Once rehab is done, the penalties will reduce to like 18% or something, which brings my owed amount down to like $25k-ish. You should get the specific terms in the rehab paperwork they send you. Read it carefully!