Impossible to say without seeing what monetary value the membership ads. Any forum with 50,000 membership has great potential, but are they realizing it?
Might be that the culture of the mebership dictates that no comercial presence be on the site.
Is there an industry(s) associated with the theme/topic of the forum, that lends itself to internet commerce.
Is the forum the only or main section at the site? Or is the forum only part of it, and other revenue opportunities are realized elsewhere on the site?
What is the mebership-total/thread-count/post-count ratio? Many sites have humoungous mebership totals with very little activity.
Take DNF: A fairly active forum. Members: 7774 Threads: 52,760 Posts: 357,379
6 threads per member 46 posts per member Check that against one of the forums your looking at. Don't forget DNF incentifies it's members for posts ($DNF)
Take
www.Bimmerfest.com/forum Members have lots of money presumably.
Members: 14,144 Threads: 62,785 Posts: 799,074
4.4 threads per member 56 posts per member Nice activity but they have 1 banner ad spot. And it looks like i just got stuck there. But there is revenue generation on other(non-forum) sections on the site. So which forum is worth more?
Obviously there are tonnes of other things to consider as well