When I settled they offered me to pay 50% of the original debt (the debt had actually gone up to twice more of what I originally owed, so in theory I settled for 25% of the final amount). At this time I didn’t know I could negotiate an amount so I agreed, but I think it was reasonable.
There were a couple of articles that I found helpful, I just can’t find them right now. As soon as I do I’ll post the links here. If I remember well, if these are charge-offs you are trying to settle, I believe you can start as low as 25% but I’m not sure.