Sephora sells a similar set at $8 for 8, but I've got them all beat at around $0.13 per incredibly easy-to-make tie.
What you need:
- A few yards of fold-over elastic band, 3/4 - 1" thick (any fabric store will carry this, but I got mine here)
Measure and cut the elastic in 8-10" increments, depending on the thickness of your hair. Mine is lamentably on the thinner side, so I can get roughly 3 loops around my ponytail from an 8" tie. Experiment to see what works for you.
Fold your now cut elastic in half and make a loop.
Pull the ends through the loop to form a simple knot, leaving approximately a 1" tail.
Tighten the knot and repeat all of the steps with the leftover elastic.