@tuffymon The pricing of the slaves is based on their abilities, looks, body, blemishes etc. The only thing the two figthers could do was fight and one was willing to go to bed, but had no skills there. I would not be surprised if Wendy have been properly trained to pretty much be the perfect wife with training similar to what we see from mangas with queens in training, but swap politics for bed skills.
Also, potions have always been at a premium no matter what world you are in, since it takes time to gather materials and craft them, so have to include market price for materials, cost of labour to gather them and make them, then add 10-20% on top of that to make sure they make a profit. Seeing as the pharmaceutical guild sells potions as well, but apparently at a steeper price then the alchemy guild while also being more shit, we know that the adventurers guild much rather prefer to buy alchemy guild pots then pharma pots.
End note: The entire idea why he chose the skills he did was to make money fast enough to make it op, so once he skills up to make better potions, can probably make 10s of millions of gold a day if he wanted, but most likely he will work for a day, lounge for 6 if not more, repeat.