Having never used the Spa or therapy pool, I'm sorry I can't help you. But, if you go to HAL's web site, and click on
http://www.hollandamerica.com/onboard/spacruisefitness
or go to HAL's website and click on On Board Experience, Greenhouse Spa, it will give you an idea of what it's all about. Sorry I have no idea about the prices or weekly passes.