I too have found National to be more expensive lately for most of my business rentals (midweek). For example: I'm going to LAX on a Tues-Thurs in June and National comes out at $169.06; Avis is $110.16--both using my corporate contract ID.
My only National rental so far this year was a weekday reservation in DTW, which was within a dollar or two of Avis and Hertz. I always prefer National if the rates are within +-10% of the other agencies, as the vehicles are newer and the pickup/return process is less hassle with Emerald Club.