Go to Office Depot - purchase a sheet of foam core (1/4" thick) - while not rollable like a true battlemat it only costs 1.99 and is usually marked with 1" x 1" grid of very light ghosted lines. Buy a light grey permanent ultra-fine point sharpie and use a yard stick to darken the ghosted lines.
The foam core has a plastic coating which you can use Vis-a-Vis markers on.
Viola battlemat for cheap. Also note that the foam core comes in a variety of colors so you can have one for grass/forest(greens), one for rock/caves (grey), one for water (blue).
Later,
Greg V.
Natural Twenty Gaming
www.naturaltwenty.com