I just installed wpjobads to our Wordpress blog which doesn't have the 50% cut but was a $50 license. Our site gets 50k PV in a localized niche and we are hoping to charge at least 10-25$ per ad.
I just stopped in my tracks today posting an ad on a site that wanted $99. Had they asked $25 I would've gone ahead.
I have no idea what their readership is like, and I didn't care enough to bother figuring out if it was worth it. I was willing to chuck $25 without thinking about it if for no other reason than to support the site. $99 - for me - was beyond what I felt like wasting today.