I used 100% cotton denim fabric, strong poly-web binding, heavy-duty plastic buckles and had the vests screen-printed.
My total cost for 110 vests of varying sizes was about $1100. So each vest was about $10. I was able to purchase materials in bulk which saved money, but the cost of screenprinting was a big expense at $4. per vest.
You could use less durable materials and fabric to save money. You could also use iron on transfer letters or cut letters out of fabric and sew them on. You could use bias tape or other ribbon instead of webbing and buckles to secure the vests by tying them at the neck and belly instead of having the buckles.