That is the whole point. It is extremely rare for anyone to post an item at the price they think it is worth because it attracts less sales. Instead they post them at a price that they think people will be happy to pay.
Therefore the price listed on the site has nothing at all to do with what the writer thinks it is worth.
It's called Supply AND Demand. Not Supply OR Demand, not just Supply and not just Demand. The obvious answer is that the Price Point is a compromise between the two. Would I have been happier to pay less for my 5E books? Probably. $50 is kinda steep for it's content IMO. Are people "happy" about this price point? I don't know, you'd need thousands of dollars of market research to find out. I think by and large, you'd find most people are simply
used to the 50-a-book price point, not necessarily happy about it.
More than that though, the notion of being willing to buy a product at an expensive price (say $10) but not being willing to buy that same product at a cheaper price (say $2) is absurd and flies in the face of economics. If you genuinely think that way, you aren't anyone's target audience - you are a statistical anomaly.
This makes so many bad assumptions it's really not worth addressing. So, unless the product you're selling is gasoline, water, or milk, your product is probably
not exactly the same as someone else's.
There are many RPGs on the market. Many of them are cheaper than D&D, but many of them are also
not D&D. Clearly, they are different, and thus, warrant different prices. Even for the "Generic Fantasy RPGs" genre, there are a lot of cheaper alternatives to D&D. Yet D&D is still one of the top sellers. Your focus on price is missing the mark so badly I wonder why're you're even shooting for it. D&D offers a lot of things that another product might not. Developer support, errata, communication, high editing standards, high functionality, broad appeal.
So again, unless you're selling gasoline, water or milk, there's no such thing as "the same product" unless you're under the impression that a Ford Focus and a GMC Yukon are the "same product" because they are both cars.
Clearly this
is not this
so even though they're "the same" in so much that they both represent a woman, sitting, while possibly smiling in front of some scenery, I suspect one of these you you pay more for.