Provide data fields to allow travelers to input actual expenses which can be used in expense report preparation.
It's an interesting idea. You could keep track of expenses now by adding a comment to an item.
Very few people are using Planapple for business travel right now, so we don't really have plans to add direct expense reporting features.
If you're looking for expense tracking, Mike recommends Expensify (and uses it himself). You might also look into TripIt's expense tracking integration -- they're owned by Concur, which is a big business travel support company. (Planapple isn't associated with either of these companies, but it does have the ability to sync your TripIt itinerary.)