Step 1: What You Need!
A dry erase board of any size (about 8 bucks)
Dry Erase Markers
Eraser and Eraser liquid (Together with markers 7 bucks)
Spray Paint (I used black in my but in retrospect gray probably would have been better so lines are not confused with walls) (Just over a dollar for a big can)
1 inch masking tape or duct or and sort of tape (A buck)
Cost: about 17 bucks although you can probably find cheaper.
Step 2: Drawing the Lines
Next get out your 1 inch masking tape and put a piece horizontally across the top of the board
Now a millimeter or 2 (You do not need to be meticulous about the measurement) below it put another piece continue until your horizontal tape pieces with small spaces between them cover all the way down your board. *
These pieces should be as close to parallel as possible.
Now get out your Spray Paint and spray a few inches about the board and cover the whole board.
Wait until the spray paint dries and then remove the tape, you should now have horizontal lines going across your map.
Its quite likely that some of your spray paint has "bled" giving the grid a distressed look don't worry about this because in case you're unfamiliar with Dungeons and Dragons the maps you buy also have a distressed effect on them.
*Conserving Resources: If you don't want to waste all your tape you can save by using only 2 or 3 pieces at a time, spraying them and then moving them down. If you do this make sure you cover the blank part of the board with newspaper first so paint doesn't get on it.
Step 3: Drawing Verticial Lines
After you finish you should have a grid.
Step 4: FIN!
Use the dry erase marker to draw dungeon walls and other terrain and then when you want a new map erase it and there you go.
Now grab your dice bag, monster manual and be on your way!