Absolutely free - just bring your own pillowcase (just a hint - wash it before you have it done because even sharpies run after laundering) and your own sharpies.

What I did last year was put a pillow case and 4 of the boys' favorite colors in two separate gallon size ziploc bags (one for each kid) and gave it to guest services on the first day. They have a form you fill out and then, at the end of the cruise, your stateroom host will leave the finished product in your room. I also gave a bag full of chocolates along with the pillow cases - heard that the CMs really enjoy candy