I don't do koolies, but when I decide to try something new in my small business, I shop around to see what others are charging first. My most favorite place is etsy.com since they cover just about any type of craft project. I went to etsy and just typed in 'koolies' and here was the result.
Etsy Koolies
Considering their build-cost is very low... I would think 5 or 10 is appropriate. Of course you never want to get into the habit of over or under charge.