I don't know about celebrity but rates are similar to what you might find in hotels. The charge is by item, not the load, and varies by item - that is a dress is more expensive than a pair of shorts. If you want to take the time most ships have laundry machines that would be much more economical.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014