I never said she was "better;" I'm talking strictly monetary, she dwarfs Roxanne by quite a significant margin. Firstly the rarity of Master Smith slaves not even the Imperial slave shop had one and you'd assume that would be the most likely place they would be and later [LN8] one isn't even brought up at the auction, secondly she's a consistent source of income, thirdly she gives him access to weapons with skills [LN8] for reference a Staff with MP Absorbtion goes for over 150,000 Nars, fourthly she researches labyrinth monsters and their weaknesses and plans out their strategy (e.g. focusing on farming the items that become pills).
Sherry is indisputably a more valuable asset and many slaves would use that to increase their position or effectively force a sale.
I do not deny anything you said, but all of it is subjective. I absolutely understand there's no way he puts anyone above Roxanne, but if we look at it objectively rather than subjectively Sherry is more valuable [LN 11-13] well until it comes out she can just straight up dodge the final boss of a labyrinth, but even then it might be close