Considering:
- How Ars magica is abstracting the concept of money,
- That people had a rather different way of perceiving wealth and the notion of savings (= hoarding money) and the sin of greed,
I don't think there is a right answer to your question.
For a general discussion on price of things, I would refer to this Discussion on medieval prices, which also highlight how difficult it is to have a "right" price.
Second, I would suggest the reading of "City and Guilds" as it tackles wealth, social classes and how it is abstracted within the Mythic Europe paradigm. This should help you see where your couple of skilled craftsman and shrewed sellslady fit in the social classes. Unless they have special virtue, they are just in the "average" segment. However, with their good skills they might be able to "ascend" to a higher class. This is covered in City & Guild with the mechanic of labor points as you pointed out yourself. I honestly find it too detailed and complicated for the kind of stories I like to tell, but it might point you in the right direction.
Since you did not find the answer you were looking in C&G, maybe it is because you are not asking the right question and should rephrase it?
Why is it relevant to you how much they can sell their good, and what is their margin/benefits ? It is because you have a goal in mind for them. Thus a story you would like to tell ?
If it is about social climbing, it is covered by labor point. If it is about buying/getting something, you can also abstract it with the concept of social standing: they want a better house, a larger property ? They need to become "Wealthy" and we go back to the labor point system. They want to become head of guild, again climbing the social ladder, with some story telling in it. C&G rules give a kind of scale of difficulty on how easy/tricky it is to change classes - how long, it takes, what kind of compromise you have to make (like getting the "Miser" reputation for trying to maximise you time and profit - which is strange to us, but reflects perception in Mythic Europe paradigm).
They want to acquire something extraordinary like a magical item or a longevity potion ? then it is not only about wealth, it is about having the right contacts, getting favors - this is the kind of material that SG likes to hear about, because they can spin a story for you - especially that those who can manufacture such items are rarely interested in pure mudane wealth: they might value a season or a year of secret work from a skilled craftsman more than just a few pieces of exquisit jewlery, or maybe a favour to be called later.
As a SG, when I had to deal with a wealthy PC merchant it was never about how much coins he had, but more what he wanted to do: become a patron of an orphanage to be invited in certain circles for example. He never asked me how many healing poultices a month his alchemist guild was selling, he just knew he was wealthy and could use his wealth as a tool, whereas other PCs had to rely on skills and wit.