http://www.mammothmountain.com/MountainActivities/Passes/
Also, from the FAQ:
What is PassCash and what are its benefits?
PassCash is cash loaded onto your season pass during the April Pass Sale and is good at any Mammoth Mountain operated business starting on June 20, 2009. It’s great for kids of all ages who don’t want to worry about carrying cash or credit cards when they’re on the hill. And if you load PassCash during the April pass sales period in $100 increments, we’ll throw in a 20% bonus (for instance if you buy $100, we’ll credit your pass with $120). But this promotion is available only at the time of purchase and available in $100 increments. And yes, you can use your pass cash to purchase your 10/11 pass, you sneaky devil.