This is very interesting. First off, I thought it was higher (75-80%). Secondly, this makes me wonder if I should be using my own site search script with embedded content ads (68% rev share) versus the Adsense Search (51% rev share).
Haven't we heard for years that people pay more on adwords for search? I'd rather have 51% of 0.30 than 68% of 0.10. Not to mention all the additional work.