I accidentally stumbled on a profile of his mom and her art collecting the other day while I was looking for something else, and I was also kind of blown away by the amount of money. I guess his dad became an art dealer as a way to make ends meet and was pretty successful, and when he died his mom kept collecting art with the foundation his dad left behind. Doesn't seem like they were born loaded or anything, but I could be wrong. The profile made a point of mentioning that she was a social worker and her husband was a teacher before becoming an art dealer. There's some quotes from Mike about having famous people in the house and how formative it was to discuss art with his parents growing up. In terms of the level of money spent on this stuff, it probably doesn't hurt that her son was in a successful band
I just tried the track down the article I read, but I'm pretty sure it just quoted from the Sotheby's stuff so the OP link will probably lead you to what I was reading.