I've done several settlements with Navient/Sallie Mae. Make sure that there is language in the letter that it is settled in full. They are most likely offering you such a low amount because the account may be past the statute of limitations for credit card debt in your state. This is what I've noticed with low Sallie Mae/Navient settlements.
Sometimes, there are issues after the settlement where it looks like they just applied the payment to your total balance (via your online portal, for example). So be prepared to file CFPB complaints if there are any issues post- settlement. If possible, record the call when you arrrange the settlement.
I also advise all of my clients to send settlement payments by certified or registered mail (for proof), and also use a cashier's check or money order with the file number and other info on the "Memo" line. They may ask you to send the entire amount by phone or electronic debit. Don't do it! You need a paper trail in case anything goes wrong (which has happened to my clients twice, but was resolved with complaints). Also - I am not an attorney, and this is not legal advice.
Edit - I wanted to emphasize the most important part - get the settlement in writing before doing anything.
