Nice name. As others have mentioned, I also agree that DomainForum.com would have been better. But, this itself if properly developed with some new ideas can fetch mid $$$$.
Well I think you paid end-user price. As far as my appraisal...most likely $10k-$25k. It is definitely a name with huge growth potential.
I know some here prefer singular over plural but I am not sure it's that important in the long run. I would say if you have one version and a competing site had the other then yes. But the singular in this case doesn't even resolve.
Like Namepros.com is of course plural...the singular is just a parked page.
I say if you sold it on Namepros 2,000 tops, but if your willing to hold on to it in 3 years xx,xxx. Namepros is the forum to sell it at, but is their anyone her who would pay xx,xxx here now? If so come foward and make this man rich.