I like this domain a lot. Obviousally the .com would be much better, but the .net is great as well. I would appraise it at low $xxx - to mid/high $xxx , depending on the buyer. If you find an end user, I think high $xxx may be attainable. About development, I would only develop it if you plan to spend a little time developing it. An empty forum will add absolutely no value to the domain, and possibly decrease the value. However, if you can get at least a small user base, 10-15 active, posting members, then your forum will fill up fast, and you can expect 4 digit offers. Just my thoughts,
Tom