B C Z
A parcel being "purchased" doesn't effect the price of the land. The land is still empty even if someone owns it up until something is built on the land (house, building, etc).
So there is no practical difference in the order in which parcels are purchased.
I usually wind up going left to right top to bottom (like you read a book) just for simplicities sake and to keep people from jumping ahead when we may have duels to resolve.
Note that duels are resolved before purchases, and order can very much matter here, so you should be deferring to the dueling player who passed earlier in the round.