I would guess $1000. Definitely not enough sales data to really estimate by comparable sales though. Agree with Kate's assessment, and I would add a possible use for this is as a political website. I think .PRO extension is not only good for "professional" but politics (in favor of) although I haven't seen it used as such yet!
Assuming Democrats gain more power in 2008, the PRO GUN advocates may come under more pressure. There may even be a new round of legislative attempts to restrict registration of handguns and automatic weapons. If this comes to pass, pro gun forces like the NRA will certainly be fired up and a domain like this would really fit that agenda. Lots of "ifs" in this scenario, but it's not really all that far-fetched.