Originally Posted by
sbm12
The cost basis likely isn't the issue. Many airlines have stopped allowing their lowest fares to be combined with other fares in an end-on-end manner. So there may be a $100 LON-BUD-LON fare but if it is tied to the same ticket as WAS-LON-WAS it could jump up to $300. Booking as separate trips addresses that issue but can create other issues at the same time.
You are right, there could be several other factors involved as well. Depending on how OP splits up the routing and how long the layovers are the computer system may also think that the ultimate destination has changed. I vary rarely have luck booking itineraries like this, but yes it is possible.