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 × spell level (0-3rd) × level of the caster as stated in Creating Potions.
So at the following levels a Potion of Cure Light Wounds cost:
5: 25 × 1 × 5 = 125
10: 25 × 1 × 10 = 250
15: 25 × 1 × 15 = 375
20: 25 × 1 × 20 = 500
It seems that making a profession of brewing potions doesn't seem prosperous.
That is to keep the magic items, in general, balanced.
A lot of spells scale better than Cure Light Wounds (which, in fact, scales terribly) and if brewing higher caster level items of lower level spells would make those potions more desirable than actually using high level spells, there would be no reason to make items using high level spells.
I will give you an example. A wand of snowball at CL 1 costs 750 gp and does 1d6 damage as a ranged touch attack. A wand of scorching ray at CL 3 costs 4500 gp and does 4d6 damage, also as ranged touch. If we take that snowball wand and increase it to CL 4, it will cost only 3000 gp to deal 4d6 damage, much cheaper than the wand of scorching ray.
This scenario can be seen on many magic items, not only wands. But if you make that comparison with cure wounds spells, it will show you that multiple castings of cure light wounds will net you a much better hit point per gp healing value than any higher version of the spell.
Example, a CL 1 cure light wounds potion costs 50 gp and heals 2-9 hp (average 5.5), a CL 3 cure moderate wounds potion costs 300 gp (6 times more) and heals 5-19 (average 12). While a potion of CL 3 cure light wounds would cost 150 gp and heal 1d8+3 (average 7,5). So two potions of cure light wounds will always heal more (2d8+6) than a single potion of cure moderate wounds (2d8+3), with the downside of requiring more time to heal that same amount of hit points. And if you make the lowest CL cure light wounds potion (1d8+1, 50 gp) you will heal much more than both options (6d8+6), but requiring six times longer to heal.
It seems that making a profession of brewing potions doesn't seem prosperous.
Even if you brew the cheapest potions for 50 gp, it only cost you half of that value (25 gp), it will always be a profitable profession.
Please realize that you can always create magic items using the minimum caster level for that item, as described under Magic Item Creation:
While item creation costs are handled in detail below, note that normally the two primary factors are the caster level of the creator and the level of the spell or spells put into the item. A creator can create an item at a lower caster level than her own, but never lower than the minimum level needed to cast the needed spell. Using metamagic feats, a caster can place spells in items at a higher level than normal.
