I just got back from the 2 Day M School last week (there is a write up from another attendee on the M5 board). I can't comment to the cost vs an HPDE day, but what I can say is that the fundamentals you learn at the 2 Day school should help you quite a bit when you do HPDE days. The other nice thing is the wear and tear on a car that isn't yours. They have the new M5 and M6 in addition to the M3. One of the guys in my class (also who wrote the article) put his M5 into the guard rail and the cost to him was ZERO. He got to really push, learn, and make mistakes without the fear of damaging HIS M5.
The 2 Day program is first class and I'm now looking forward to saving the for the Advanced school. Put it this way: have you seen or read one bad review on it? I haven't seen one yet.