In my understanding, that should be an average cost of the...

In my understanding, that should be an average cost of the purchases. As the range of the price is between 0.525 and 0.55 on Sep 20, it is not hard to gauge what his purchase packages could be.Because his average cost is 0.5321, he definitely purchased the shares at 0.525 or 0.53 or both. If he purchased in two prices, we can solve a linear equation in two variables. I tries it, the possible combination could be:
(0.55,0.525)=(10650, 26850);(0.545,0.53)=(5250, 32250);(0.54, 0.525)=(17750, 19750);(0.535, 0.525)=(26625, 10875);(0.535, 0.53)=(15750, 21750).He might purchase the shares at more than three prices. It is not hard to figure them out.

That is what i understand the consideration of 0.5321. AS the other poster Stephie pointed out it could be 0.53 plus fees. That is also a possibility though in my opinion as a co-founder of Bombora, brokerage fee could be negligible for Mike.