Hybrid cars are a hot topic right now, and forums about it would most probably get a lot of interest. Undeveloped, would probably be mid $xx, but if developed could be high $xx to $xxx.
Let me add that a well developed and Administered hybrid automotive forum can be worth as much as the amount of effort put into it and perhaps even more.
I believe the issue of dependency on oil is and will remain one of the biggest hot-button-issues for a very long time to come, couple that with the fervor of alternative fuel advocates and it presents a combination of initial interest and stickiness to keep members around.
IMO, people are interested in hybrid cars (incl. me), so the interest/pop. factor is big and since you've got both versions of the name, its fairly unique. So I'd value it between. high $xx to low $xxx.