(Disclaimer: I am a bit biased as we operate a luxury ski accommodation in Niseko.)
For the amount of terrain and powder options in Niseko, it is an amazing value. For example, in the upcoming season a 6 day all mountain pass is under 28,000 yen (less than $250 USD).
Niseko also features some of the world's best night skiing conditions on (reportedly) the largest maintained single night course in the world.
That adds up to a LOT of skiing for less than half the lift pass costs of many North American resorts.
Dining and entertainment prices are no more expensive than other ski resorts around the globe. A very nice dinner is easily available for 3000 yen or less. If you just feel like a bowl of noodles or a rice bowl, you can fill up at lunch on the slopes for 600 - 700 yen. (You can barely buy a plate of chips / French fries at a number of resorts for this much.)
Perhaps the best hidden value of the resort are the number of thermal hot spring spas scattered through the area. You can soak away your aches and pains for a mere 500 - 700 yen.
Niseko has both Japanese and western accommodation options, including a number of self-contained apartments which are ideal for long stays, and often no more expensive than a hotel for week stays.
Japan has a negative and undeserved rap as being expensive. You may have heard of $100 sushi dinners, $200 Kobe beef steaks, and $1000 watermelons. While these niche items certainly do exist, these sort of prices are in no way the norm, and have been sensationalized by western media. With exchange rates the way they are now, Japan is more of a value than ever. The JNTO has even started a program called "Affordable Japan" to dispel the image that Japan is an expensive place to visit.
http://www.jnto.go.jp/eng/arrange/attractions/affordable/index.html