I don't think it's a good ideea to do a joint venture involving adsense. You have to trust the person and really know him/her before embarking on such a journey. How will you know that he is reporting you truthful stats....he may say that you earned 2$, when in fact you would have earned 5-6$? This is a risk not worth taking. Just try another advertiser.
Instead of asking for a partnership in AdSense where you would have to split your earnings, why not try other ad networks? I have tested Kontera, Chitika and WidgetBucks. So far, my tests showed that Kontera pays the best (second to AdSense) so why not try them? Tony Alves
I definitely think that using other services might be your best bet, but depending out how much the other services pay out, 80% of adsense could be more than any of the other services. PM with the links to the sites and I might consider.