Lights.
Lights are an absolute rip off.
Think about all of the LEDs in a television and the controllers required to power them.
A 65 inch OLED TV has 8,294,400 pixels (3840 x 2160).
Each pixel requires 3 LEDs (R,B,G) in OLED
Total LEDs 24,883,200
Won't even think about 4K or 8K and we'll throw in smart TV and audio for free.
Cost of TV $1,500 - 2,000 depending on brand, model, etc.
Now I realize this is apples to oranges, but surely a reef light with say 72 LED's total should sell for a lot less than what we see today. Heck, even the bargain lights like Nicrew which sells for about $200 are overpriced.
TV cost per LED $0.00008037551
Bargain Reef cost per LED $2.77777777777
A close second is the cost of Skimmers and Roller filters. Go there some other time.