The question of comparing an open source solution to a big expensive app like Open View came up not too long ago when GroundWork published their cost comparison of GW Monitor vs HP Open View. Over three years, GroundWork Monitor was 82% less expensive than Open View. GroundWork is about $54/node and Open View is about $295. You can download the PDF here: http://www.groundworkopensource.com/solutions/tco-gwme_vs_hp-openview.html
This study caught the attention of Matt Assay on cnet when HP had a fit about their pricing being exposed. http://news.cnet.com
You probably saw it on slashdot right after Thanksgiving. http://news.slashdot.org/article.pl?sid=08/11/28/0857213
I work at GroundWork. We did not sign an NDA to get this document.