If you had five equally popular search engines, they wouldn't really compete prices down anyway. The number of searches (supply) would still be the same & advertisers (demand) would still be the same.
The only reason MSN & Yahoo (not to mention real obscure engines) still earn less per click is a sort of market imperfection: It's complicated & time consuming learning another system & running parallel campaigns. For an extra 5-10%, it isn't worth it for small campaigns.
Monopoly or not, they are not manipulating the market. It manipulates itself.