If you are claiming that hornbeam came from a landscape nursery already looking like the photo that you posted and cost £70, I call bullshit. If you really bought the tree for £70 in the state of development shown in the photo, then you had to go through the wardrobe to get there because the nursery is in freaking Narnia. If you're not claiming that and the photo is actually what the tree looked like 5, 10, 15 years later then you're being extremely disingenuous and comparing apples to oranges. Any vendor's prices are a reflection of what the product is worth today, not what it might be worth years later after the customer has held onto it and skillfully modified it.
I can't afford Bjorn's high-end material. I don't have the financial means. I've never paid more than $300 for any tree, ever. That's what I can afford. But, I've done enough drooling over high-end trees while window shopping online and in-person to have an idea of what the typical price range is for trees like that in the U.S. The fact of the matter is his prices are comparable to what other U.S. bonsai vendors like Brussels, Mirai, Wigerts, etc. charge for material of similar quality. It's not some Facebook scam sale where somebody buys nursery stock, does nothing to it besides sticking it in a bonsai pot, and then charges a fortune to uninformed buyers.
I have met Bjorn multiple times and it was crystal clear to me that he's 100% in the business because he's passionate about the art of little trees in containers. He's not in it for the money. Just take a look at all the YouTube educational content that he's been giving away for free for years. Sure, he's doing the Bonsai-U videos that he charges for now, but the guy has to make a living and keep the lights on at Eisei-en and no one should begrudge him that. I find it rather obnoxious that you're running your mouth off about his prices from the other side of the pond when you clearly don't know the U.S. bonsai market.
Your suggestion that we should all just go searching for buried treasure at local nurseries is insulting because you're assuming folks aren't already doing that, when every serious bonsai person I know has been doing exactly that for years. The overwhelming majority of my trees were nursery finds. I didn't just pick the first tree I saw. You have no idea how many trees I picked up and examined closely in order to find what I've got. Those trees are representative of the most promising bonsai material that was available in the landscape nurseries in my region. I have a few pre-bonsai purchased from a bonsai nursery. I wish I had more of those, but cost is prohibitive. It is what it is.