A couple weeks ago, two 1986-87 Fleer Michael Jordan Rookie Cards sold for $738,000 each. That's a high number, but not nearly as high as a Mike Trout rookie sold for auction this summer. Prices like this, against a larger uptrend in sports card, immediately arise suspicions and usher in questions of whether this is a bubble or what a piece of cardboard should be worth.
One way to think about these values is to look at the art market. The greatest athletes of our time, such as Jordan, could be thought of as artists in their own right - geniuses that come around every so often. In that context a rookie card can be thought of as a way to collect that artists "work". I'm sure I'm going to evoke a ton of cringing from art snobs, but I bet Van Gogh's crossover was weak and Dali couldn't dunk.
Look at Picasso - an artist who everyone knows and who is said to have produced a ton of work (estimated at about 50,000 pieces in total). His most valuable piece of artwork, La Reve, sold to Steven Cohen (the famous hedge fund manager who also is now investing in sports cards) for $155 million in 2012. I'm sure it's estimated at a much higher price tag today. Picasso has a ton of other works that are well into 8 and 9 figures.
There are currently 316 PSA 10 Jordan rookies today, and 18,000 submitted for grading in total. It's probably fair to say Jordan produced just as many pieces of "art" as Picasso if you consider his rookie card to be just that. In this context, $750,000 could be massively undervalued - and it all depends on what people who have money will value culturally in the future. I think Michael Jordan at least has a seat at that table with Picasso, Ferrari, and Pokemon.