I believe you will actually pay more on the ship than on land. However, a lot of cruiselines offer discounts on port days. Watch your daily paper for the specials.
Mary Lou Scanlon
NCL Pride of America April 24, 2010
NCL Epic February 12, 2011
RCCL Allure of the Seas - September 18, 2011
Celebrity Eclipse - February 11, 2012:
RCCL Navigator OTS - February 9, 2013
on average expect to pay about double from what you pay at home if you get a spa treatment on the ship. Look for the specials that run on port days.
Unless you go to a chain hotel i wouldn't recommend getting a massage/manicure-pedicure on the island, sanitary reasons.