I think they're slightly less if you buy them in advance online than if you buy them in person.
Well, you can buy them in person as well and get the same online price, as long as you do NOT purchase on the same day of the party, so buy in advance. Same day tickets will cost a bit more.
Besides the APer discount price, which is still a stiff $40 for what these parties offer in return (read my P&PP review on this section of MC), the other way you could get a discount is by being a CM. CMs get tickets for a price of about $25 to selected parties. Now that should be the regular price for guests for these parties.
I agree what they charge for the parties is too much, but if you get there at 4pm when they start letting party guests in, it gives you a full 8 hours in the park since the party goes until midnight. In the end, that's what justified the price for me last summer as it was still cheaper than a regular admission and you get almost a whole day's worth of time in the park. I went for the rides, not the party. I could care less about the parade and the fireworks and the "special" stuff they were doing. Just let me go on the teacups 3 times in a row as a walk-on, Pirates a walk-on and a 5-10 minute wait for Space Mountain and I'm pretty happy.
I was in the MK the other evening, and it was slow like a MK E-ticket night. I was told (by someone who has been to 3 incarnations of the party, I was only at the first) that this was the worst one (and he went over many reasons why).