Elmer,
A good starting point is - a piece of an old indoor/outdoor carpet. Primary purposes of the mat are to keep you out of the dirt, and to keep you from slipping around while firing. A piece of carpet does those just fine. What the carpet lacks is padding to make lying on the ground comfortable. This can be fixed by laying a thin foam camping mat under your "mat".
Save your money at first for loading supplies. Good, used gear can sometimes be found at matches. Or, look on boards such as this one, or at long-range.com or nationalmatch.us.
DISCLAIMER:
I am an Admin at both long-range.com and nationalmatch.us. However, I derive no benefits (monetary or nonmonetary) from being an Admin.