I think now a days those other extensions are starting to rank well. I have a feeling extensions like .ws .co and various other ones are starting to rank in google more and more as Google has decided to level the playing field. Even though the SERPS are complete garbage right now – this would be a good time to pick up this domain and see how much it can make over the course of several months. My only fear would be if they roll back on Penguin which is very possible then the .ws domain may be much harder to rank.
Right now though $25-50K seems like the right offer..
I would of picked it up for 20K most likely as i feel that would be an easy price to sell it for developed if it indeed flopped.
Good luck on the sale – but like the other person said – keep it and at least put some content on it and develop it a bit – may even be easier to sell for 50K your asking price.