this is a big problem, with the we will charge you extra scenario.
The way United have handled it does seem a pretty good way, then reimbursing their own traveling fans with the difference.
So Sevilla charge £90 for an away ticket. United fans pay it. Sevilla would normally charge £30. So United will look at their own ticket pricing and add the extra on top of the normal ticket price, to reimburse their own fans, when they sell tickets to Sevilla for the return fixture. So adding £60 on top. Sevilla fans pay it or don't go. Either way i can see Sevilla fans being pretty cheesed off, that they have had done to them what their own club did to United. Maybe that puts an end to it.
When people stop going because its too expensive, maybe that is when UEFA will listen.