I think there are some widly unrealistic assessments on here. This match is live on television -it is also not that cheap to buy tickets at a time when many are tightening their belts.
I remember us only getting around 25,000 when we played Spurs (2003?) in the Cup and won 4 - 0. We are a third division side - how many other third tier sides would fill a stadium with 32,000 people?
We have done very well to sell out just a few days after getting a huge gate for the MK match. It is great and will be a superb atmosphere.