I really dont see what the issue is not opening the Kop again. The game will attract what, 15k max (with a 2/3000) coming from Sunderland. There is no reason to open the Kop. Throughout summer people were banging on about how inadequate it is, the facilities are poor etc etc. so why bother opening it when we dont need to. More atmosphere will be generated with 2 pretty full stands than 3 half empty stands. And if a kop regular doesnt enjoy sitting in one of the other stands they need their bumps feeling (i say this as a kop season ticket holder - due to the fact no one i go with will tip up for the south!)
The club could make it cheaper perhaps, but i dont think it'll make much of a difference. Its a school night so not too appealing for families, United will change the team again (and will most likely look to lose (i know a member of the first team staff who was gutted we got through!) - personally would rather we tried to win these games!) and we've Liverpool a few days later, which is what most non season ticket holders would rather save their money for