If it's generic and has no potential TM issue attached to the company that's interested. I would let them sweat a bit, then respond with the google stats and adding something to the effect of how valuable that particular domain is to any company in that vertical and the new owner would have a head start on others, online. That domain can be the difference, for a company to either stay on top, or get to the top!!!
With that said, be respectful and counter offer with double - I say $6k at least!! Don't low ball yourself...but, like others have said, if you need the extra cash, pull the trigger, but don't be desperate.
I once got an offer on a dropped domain for $200....I countered with $2k and ended up selling it for $1k. In the back of my mind, I still know, that I could've sold it for more, but I'm glad I did it then and have no regrets!
good luck and be sure to let us know what the outcome is!