Scott, a store bought string should cost you about $9-$10 and the length should not make a difference in the cost. If you get a string with a loop for the top limb and a bowyers knot for the bottom limb you can make the adjustments for higher or lower brace height by moving the knot. You are limited to only an inch or 2 of adjustment if you use a string with 2 loops.
A string is very simple to make and if you are going to make bows, making your own strings is a natural progression. For what you pay for a store bought string you can make probably 10 strings if you buy the materials.