Another of my players wants to create and sell spell formula for foci and sell them to his talismonger contact. He created a force 4 power focus (it took 4 days)
Hang on - if it took 4 days, the [formula Force] was 4, and those 4 days would be followed by an (Artificing + Magic [formula Force]) v. (formula Force + telesma’s Object Resistance) Opposed Test,
without being allowed to use Edge, with the number of net hits you get being the actual Force of the focus.
So for the actual force of the focus to be 4, he'd have to reach the 4-hit Limit while the item got 0 hits on 4+3 dice (the lowest Object Resistance being 3 for things like hand-carved wood) - and the chance of the second thing happening is only 5.85%. You'd have a 20.48% of ending up with a Force 3 focus, 30.73% of ending up with a Force 2 focus, 25.61% of ending up with a Force 1 focus, and a 17.33% of getting nothing for your efforts - and that's only if you're 100% certain of getting those 4 hits.
Now, if we assume Magic 6, Artificing 6, and a Specialty in Crafting the type of Focus that you want, you'll have 14 dice and a Limit of 4, the focus has 7... assuming I'm inputting this right on anydice, that's 27.84% of nothing, 25.44% of an R1 Focus, 26.35% of an R2 Focus, 16.05% of getting an R3 Focus, and 4.32% of getting an R4 Focus. On average, that's about an R1.4357 - and if you only want the R4 Focus, it'll take you 23 tries, or 92 days, on average (if you'll settle for R3 or R4, it's 5 tries, or 20 days, on average).
Also, keep in mind that if you like the result, you'll have to spend [Force] Karma to finish the Focus, which means your alchemist (who I assume has a karma-heavy character advancement route) is basically spending 1 Karma per 4500 nuyen - if you don't allow your characters to trade nuyen for Karma, he's basically hindering his own progress for a quick buck.
Lastly, if this is a chargen contact (I'm assuming you don't make it particularly easy to raise a contact's Loyalty Rating in-game), the highest Connection it can have is 2. So that contact is going to have a hard time actually finding someone to sell the focus to. And of course, with a Loyalty Rating of 5, the player is opening themselves to plot hooks like "Your contact got kidnapped! Go save him!".