One owner is one owner,...whats the problem.  If your price for one owner search is $25 then it is on you if the current owner was vested ownership in 1960.  IF you find the Last DOR is 40 years back then the client has the option of paying for a 40 year search. 

But the real question is if you do a current owner search and its more than 20 years can you bill for a 20/40/60 year search.  I would say no... set your 1 owner search for a 40 year search if necessary and eat the extra 60 years.  I  mean honestly, you are advertising billing for the "10 yr search", which is norm and not adding the few extra pennies to cover those few searches that go beyond the average.

 Allen Bruce