Hey C,
I'd like to start my response by pointing out, if you're trying to aquire links from sites that you feel have been generated for content purposes only, then surely it would be better to target higher quality sites? Sure these sites are working now, but it's better to invest in real content on real sites in the long run, as the effectiveness of this epidemic of low quality guest posting is going to come to an end soon.
However, I'm not one to judge and we've all been there SO to answer your question - When checking outbound links I tend to use either screaming frog or xenu then dump the export into Scrapebox. I check for anomalies and whether the pages that the sites are linking to are cached (occasionally PR as well). From there you can export the links into your favourite backlink checking tool (LinkResearchTools / OSE API / Ahrefs) etc.
Other Outbound Link Checkers:
Xenu
Screaming Frog
Wintzell - http://www.wintzell.net/seo-tools/domain-outbound-links-check.php
Bad Neighborhood Tool - http://www.bad-neighborhood.com/text-link-tool.htm (Checks external sites as well so can take a long time)
Not sure what the averages are, but I think a bit of common sense goes a long way in this instance. If the site has 100 guest posts, and 3+ outbound links in the author bio per page + sidebar links, you know it's overkill.
On the otherhand the site accepts occasional guest-posts and consists of high quality content, then they may have the same number of links as most good sites will link contextually to relevent sources. So averages / figures don't really help in this instance