How can you possibly prove this? The observations I have made tell me that for some stuff people will buy because it's a good deal (usually non-essential luxur

How can you possibly prove this? The observations I have made tell me that for some stuff people will buy because it's a good deal (usually non-essential luxury stuff) but for things they actually need in their day-to-day, they pay extortionate prices because there are simply few or zero options.