That's not how fare pricing works. It's not like a flat supplement on the sector concerned - that would be very easy to do and to computerise. Furthermore existing taxes have to be recalculated for any changes or currency variations. Not all tickets are combinable. So it's not a simple matter when there are perhaps a hundred fare options on this trip. So it's a manual process, not necessarily a complex one, but it requires skilled agents to work through the options and check all the details. Each fare basis may have 3 pages of details to wade through.