Way back when I used to get a piece of light canvas from Joanns (or any other fabric store) and mix up a bucket of warm water with just enough paint in it to dye the fabric. Make sure its well soaked and then "wring" out before hanging up to dry. It drys with lots of light/dark spots that give it a very mottled look. After drying I would come back and use a sponge to daub on various other shades (darker greens and light browns) to enhance it even more. They always turned out pretty great and you can make them as long as the piece of fabric is you get.
I went to the local JoAnn fabrics and got material in green or brown or grey and cut it to 4'x6' to make the play area and then used more material in different colors to make terrain cut outs for hills, forests and ponds so they'll show up better against the playmat when I was running Warmachine tournaments. Spent maybe $40 and an afternoon with a pair of scissors and a ruler.
4'x6 is about the smallest size I would use for CAV. 4' x 8' would probably be better.