Why does it matter what a reasonable price is? I mean, it matters in the downward direction, in terms of trying to capture back value that would be captured away from you if you just stated, bald-faced, what you valued the item at.

But in the upward direction, it's actually better to not know the price other people are paying, I think, because anchoring effects can be shockingly strong. Consider that a night at the movies can cost more than a year's subscription to some streaming services. If you didn't know the price of movie tickets—what other people are willing to pay—would you consider them to give you as much utility as a year's movie streaming? If not, then the anchoring effect is screwing up your sense of how much you're willing to pay.

Try this, sometime: decide an actual dollar value for how much enjoyment you would extract from taking up some fancy hobby (say, archery) without knowing how much the equipment costs. Then look up the equipment cost. The proper response should be to balk if the cost is higher than your estimated value. But what people almost universally do instead is to convince themselves that there is at least as much value in the item as there is cost.

Some video games cost $2. Some cost $70. The graphics might be fancier on the $70 one, but the amount of time you'll actually spend engaged with the game is likely completely uncorrelated to the price. You buy the $70 game because you assume it's worth it. You buy the $2 game because you also assume it's worth it. If you didn't know the prices others paid, you'd likely be completely unable to guess which was which.