you often find footprints of discontinued tents at a lower price than you can but the raw material for.
The reason is because for a shop that is dead money ..
However a foot print is designed to work as that not as a tarp.
Apart from the cut the tie outs may not be strong enough to cope with wind the material also may not be all that waterproof when kinetic force is applied.
(that is wind blown rain or just large drops)