During low season, one might find an OK/poor mileage run (cost/mile wise).
During the summer I would be VERY surprised if anyone finds a last minute MR with a good cost/mile.
A very good MR gernerally requires two things: An excellent fare from A to XYZ, coupled with an extensive routing available on that fare /from XYZ. The chances of these co-inciding last minute in peak season are very low, except for mistake fares.