depends upon which ship you are on. Princess has self-service laundry rooms on each passenger deck. Most cruise lines offer a fill the laundry bag special at sometime during the cruise, stuff as much as you can in the bag for one price.
What cruise line? I know that HAL has laudry rooms.
Take your own soap, and dryer sheets. Also, the wrinkle release is great!!
Washers I think are $1.00- $2.00 dryers about the same. you can do lingerie and quick dry things with shampoo in the cabin, take a couple of bungie cords to string up in the shower.
conserve space by putting the liquid soaps & wrinkle spray in smaller bottles. don't forget to seal in a tight baggie. I aways double bag in case of a leak.Toward the end of the cruise the laundry is very busy. get it done early or late on the next to last day.
I haven't use the services on board but others have reported that you can send your laundry to the cleaners on the ship and for about $15 all of it's done, pressed, and folded. Sounds like a good deal to me.