Value is something that motivates people to do work (that creates value). It's like bartering (I'll fix your car if you mow my yard), but currency makes work fungible (I'll fix your car for $80, and then pay another guy $80 to mow the yard). Key to this is understanding that someone might be willing to cut the yard for $75, and someone else might do it for $60. Probably no one for $20 though.
Speculation, which you lean heavily on in your example, is essentially just gambling. (I'll get my driveway fixed up for $120, and -educated guess- it allows me to fix two cars for $80 each).
Value is found by everyone voting with their currency units on what the value of any given thing is.
99% of the time when you are confused about why something has low value, or why something has high value, you can dig into the market around it and find out why and it's almost always pretty logical (and if it's not, congrats, you can make money on fixing it). This is also fuzzy around the edges, it's never sharp lines. But the shape is consistent on the whole.