This is actually a situation we have found ourselves in many times.  We have one client in particular who wants all open and discharged mortgage information.

Frankly, it's not usually a big deal.  When you get back 40 years mortgages are few and far between.  IF YOU BUMP UP AGAINST A SUBDIVISION FORMATION I'd stop there.  We absolutely will not go behind a subdivision unless the client is willing to pay substantial $$ because lord knows from how many directions the property came into existence and how many mortgages the developer had in place.  (Why in the world they care about this, I don't know.  If there's a warranty deed in place, it's up to the title company to insure that past mortgages are discharged or assumed.)

We don't charge more for complete mortgage searches unless it is REALLY a messy property, then we request more $$ one order at a time.  (And we usually have to look up mortgage info the same way you are, especially in back of about 20 years).  It's much easier to ask for an increase when you have solid facts at your disposal (having begun the search), such as:   "We need to charge X dollars for this one because the owner 25 years ago was a slum lord and has over 100 mortgages against his name -- all of which we have to look at to ascertain if it applied to our property.  We can stop there for our usual price, but if you really need us to check out that slum lord, we need to charge X.

Alix