One of the D&D dungeon boards I tried recently was a home cooked dry erase board and my prototype works really well.
I made mine out of 3mm thick MDF because its sturdy enough to take a knocking from players, deep enough to get your fingers under and has the benefit of being able to be lifted and moved with the miniatures still on it. Great for when you walk off the end of one board, and need to place another tile.
Basically grab some 3mm thick MDF and cut it into exact square pieces- small enough to fit in your gaming bag or minis case. You want this to measure a multiple of your game grid- so something like 10"x10" if your doing D&D scale- or if you use modern lipped 'warmachine' bases- go with a 3cm grid (30x30cm). I prefer 3 cm myself as it gives minis with outstretched swords a little room. Less scratching.
The next step is really what sets the board apart- besides the price. Grab some cheap craft acrylic paints and paint it up to look all dungeon-ey, grassy or ye olde parchment-y. Whatever. If your crap at painting, you can spray glue a print out of a textured surface instead. After you draw your grid on with something like a Sharpie (though I suggest painting on soft grey lines instead so you can see the dry erase lines clearly)- simply laminate the surface using a roll of sticky book cover laminate. This takes a little practice to get right- but a few air bubbles won't be a problem.
Because the grid is under the laminate, the lines will never rub off. You also don't have to worry about scoring grid lines- which I can tell you now is a real bitch of a job.
Once I am finished my reversible game board will have cost no more than 20 bucks Australian to make. For that I got 9 10 x 10 grid double sided dry erase boards. Nice.
Another handy tip- if you accidentally put permanent marker on a dry erase board, scribble over it with a whiteboard marker. You should be able to wipe it off.