I didn't read all the posts here but I'm in the middle on this one.
Milk for example, seems to fluctuate a lot here. I'll go to one store and it will be over $4 a gallon. I'll go to Costco or another store and it's $2, both on the same day.
I'm inclined to believe market value is $4 meaning that is the price the stores would like to get but sometimes, due to oversupply or lack of demand, they drop it to $2 or $3 just to move the stuff. So, in my opinion, there isn't some evil greedy monster saying "it's a holiday week and we can gouge people" but more like "It's a holiday week and we can get our target price". Keep in mind many items, such as milk, are loss leaders. They exist only to get you into the store to buy something they can actually profit on.
If your generator dies tomorrow and you call me to fix it immediately the price is $135 per hour. If you call me next tuesday morning that rate can be as low as $65 per hour.