For what it's worth, you can pick up solid core flat panel hardwood veneer doors (sometimes finished, sometimes not) at one of the local Habitat for Humanity ReStores. There's one in Durham at the intersection of 15/501 and 40 with another in Raleigh in the area of Capital Blvd. and 440. The price may be a bit steep ($35 - 50 per door) but you won't have to do much if anything to make it work.
There really is no end to the cheap side of getting your need filled. You could take 2 x 4's and rip the sides square and build up a solid wood top for less than a solid core door. Or you could get real wacky and put the 2 x 4's on their side, drill each board, on the face, every 16" or so and through bolt the lot for an indestructable top that would last well into your grandchildren's lifetime and still cost less than $100. Any damage that occurs over time can be spot filled with 'bondo'. I really would stay away from MDF or Masonite (HDF) unless they're thoroughly finished and don't plan on breaking through that finish.
BTW, did I miss you saying how long the run would be? I'm assuming 8' or 7' run, at the most, when I give the advice above. You might want to also consider a 'backsplash' if that's sheetrock on the wall behind the cabinets? That's where I'd use the plywood, for anchoring storage racks, etc.
Dean