A 20% margin (3M buying price to 3.6M selling price) for collecting an entire set seems low.

Not to mention the commission to the auctioneer.

The challenge of conserving a rare complete set of 46 woodblock prints poses the question

How to move Mt Fuji (and then keep it stable)?

That's severely underrated. There need to a few more zero's at the end. This was a landmark event which started modernism.

Japanese prints aren't like Western paintings that are one of a kind. The prints were mass produced. The woodblock in "Japanese woodblock prints" means that they carved the design into a bunch of wooden blocks and then stamped them on papers. It meant they could stamp out hundreds of prints at a go. The artists made them for normal people to hang on their walls, like movie posters in a college dorm. A good museum gift shop in Japan will sell you genuine prints for like $300. Also Hokusai was a workaholic so he probably signed a bunch of prints.

I had the same reaction but at the bottom on the article they say that there are 2000 to 5000 prints in existence so they seem less rare than I thought.

That's individual prints .. that may be true and yet still be very very few full sets of the different prints in existence .. like that elusive hunt for the last baseball card to complete a team when only 5 were made despite 100's of every other player being printed.