We have encountered the same thing.  We handle it by charging for the number of owners we subsequently search to achieve the 24-month chain, or to find the PMM, or whatever.  We advise our clients that if it ends up a higher level search, for example, if a current owner is ordered but we go back 20 years to find a PMM, then the price of the 20-year search will be charged.  Of course, we balance it with the complexity of the search.  If we go back 20 years but there is little adverse, we would just charge the current owner price.  Once we had a 40-year search ordered but it turned out to be a current owner, so that's all we charged for.  If a 40-year search ends up taking longer than a 60+ year search, we ask for more money.  We try to be fair.  We set our prices for a median complexity of search.