once you factor in the cost of insurance on top of all the other fees and wear items, I think the cost is about the same. you get slightly more track time doing HPDE, but you'll probably learn more from the M school instructors and you can't beat the peace of mind of using someone else's car.
I did the 2-day M school. 100% worth it, and I think it's a great value once you get the 20% BMW CCA discount. especially when you include the free helmet and dvd video of your driving that you take home with you.