I think the best answers are:
1. A limited CMS - it's vastly easier to design something simple which would take care of 3/4 of the users, than to design all kinds of bells and whistles to appeal to 95%.
2. Better yet, a really good tie-in with an existing SIMPLE CMS...and open source one, which is supported by the author or a team here on an ongoing basis.
The second one would work both ways - that is, it would be good for the CMS community because when they need a forum, they could go with XF knowing it ties in easily....
I'm not familiar with all the CMS's out there, but I tried CMS Made Simple on a couple projects and it fits a nice middle ground between the others.
http://www.cmsmadesimple.org/
To tie into XF, I think mostly the shared permissions and things would be needed - and perhaps a stock style or two which matched the XF default.
Personally, I need both a wiki and an "articles" system. But that is nothing like a full blown CMS.
EDIT: A price of $95 or so would be reasonable for a good wiki and articles system. So rather than getting $5 or $20, it might be better to just appeal to 100 core folks who would pay $100 each $10,000. Maybe a limited version (up to 50 articles, non-commercial, etc.) could be available for $40 or so.