Is it going into adwords keywords tool-----, typing a related keyphrase and watching how much the cost for the related terms . then adsense click will be that but divided by 2 ?
The cost indicated in the Adwords tool is for the top spots in the search engine, but you don't know if the top advertisers (those willing to pay the most) will show up in the Adsense ads. They can just turn that option off and just show their ads in the search engine. The numbers in the Adwords tool will give you a rough indication though.
you will be way off using this method. ad cost for advertisers are different from person to person. for the same keyword, one might be paying $1.20 and other might pay .45 cents. it is due to account history, relevancy of site compared to keyword (quality score), and CTR of the displayed ads. also, some publishers will get smart priced and others not. so hard to tell and you just gonna have to find out yourself. but you can use it as an indicator