Prices re out for each party. With the exception of the first party ($139/$129), it's pretty much $109/$99 through early September, then works its way up until Halloween.
Having at least 3 nights a week dedicated to parties at the MK really affects the entire WDW crowd calendar from August through December.
Party days usually have smaller crowds at MK, but it gets significantly busier when party guests can come in for the last 3 hours. The rest of the crowds are in the other parks.
Non-party days at MK get very crowded as guests all try and get a full day in with fireworks, etc.