Can someone explain why brewing low level potions at higher levels is so costly rules wise? I get the math part.

Potion Brewing cost = 25 x spell level (0-3rd) x level of the caster as stated here.

So at the following levels a Potion of Cure Light Wounds cost:

Lv5. 25 x 1 x 5 = 125

Lv10. 25 x 1 x 10 = 250

Lv15. 25 x 1 x 15 = 375

Lv20. 25 x 1 x 20 = 500

It seems that making a profession of brewing potions doesn't seem prosperous.