Long thread on this and the basic issue is it is not clear at time of purchased which type of cash upgrade one is receiving and therefore if or if not there will be a class of service bonus. See
http://www.flyertalk.com/forum/unite...h-buy-ups.html and
UA Insider's comment (post #120) acknowledging the issue.
The rules for non-cash upgrades are clear, original purchased fare determines bonus but the cash upgrades are a bit of a lotto at the moment (although UA Insider's comments provides some guidance) and it is unclear where UA intents to go with this in the long run.