Here's my thought:
I don't need 6 skills in it, let's say I take 4. That's 20 karma, 16 with Jack of all Trades.
Making a Force 3 Power Focus forumal is a Magic+Arcana (9, 1 day) extended test, that should be easy. Aracna is something every mage should have anyway, so can't count those costs.
Now spending 18 drams of reagants on it, which is... 360 nuyen? A laughable amount really.
Now I make Artificing+Magic [Force] check vs Force + Object Resistance ... which ok, could be quite a bit higher than I first thought. But something made out of cold iron is just a +3 for a total of 6... vs my 10 ... hmm, ok. You should get 1 hit, maybe 2 and with luck 3.
But yes, even Force 3 are probably not as easy as I first thought... which would have saved me 54,000 nuyen.
For shits and giggles, lets see how I'd do on a Force 6 Focus:
Threshold of 36 for the formula. Ok that's tough. Dicepool of 15+ needed I'd say. Not impossible, especially later on (and hey your current +3 Power focus will help

), but not in the beginning
Something from Cold Iron would have a 9 dicepool, that's 2-3 hits on average... lowering my results (even assuming I hit the [Force] limit) down to 4 or 3... Unless I invest a lot more into Artificing I won't even get that. So I suppose to actually end up with a Force 6 focus I need a Force 8 or 9 Formula already, which means Magic 8 or 9 too...
So ok, yes, probably not worth it.