What "other upgrades"? This IS the monetary sink for a Mage, pure and simple - and any upgrade, at all, for a mage is contingent on Karma. If I don't use the exchange, I'm frankly forced to consider the cash largely meaningless - it takes about 36 multipliers based on the table (which is ridiculous, but still) to get that much cash. So, if the group is going on runs against a highest opposing pool of, say, 16, that's 4 multipliers per run there. If we then assume they have about another 2.5 multipliers per run on average, that's 6.5. It takes 6 runs, at which point they've brought in ~117k - leaving 9k for other expenses. It takes that 6 runs, spending Karma on nothing else, to get enough Karma for this. More realistically, the Mage would save the nuyen over time while making other Karma expenditures - so let's take it to 10 runs, which brings us to 195k and 60 Karma (note that that's much more cash than the 2000:1 ratio would suggest). Enough to get the Power Focus and have a decent chunk of karma and nuyen left over, or enough to raise Magic once and have little Karma let, and a whole bunch of cash with nothing to spend it on.
And Power Foci, and in fact Foci in general, are NOT temporary. If it gets taken, the player can and WILL track it down.