When btc traded at $10, minimum volume cost 0.01*10*0.006 = $0.0006 in fees
Then btc moved to $100, minimum volume cost 0.01*100*0.006 = $0.006 in fees
Their total income depends not on minimum volume but actual volume. Minimum volume for a single order is insignificant.
And as for your thought exercise, ask yourself this: are MtGox making more money now at the current price or when the price was $10/btc?
I'm sorry but you don't understand the market at all. Let me give you one last example. I am taking historical data.
1) 2012-08-17.
Average daily price 13.26 USD.
Daily volume 221,033.9 BTC = 2,930,580.33 USD.
Their fee income is 17,583.48 USD
2) 2013-05-09.
Average daily price 111.66 USD.
Daily volume 26,894.46 = 3,003,068.41 USD.
Their fee income is 18,018.41 USD
3) 2013-05-02
Average daily price 106.49 USD.
Daily volume 234,129.17 = 24,932,099.53 USD.
Their fee income is 149,592.59 USD
Despite day 2 price is 8.4x higher than in day 1, they earned almost exactly the same income, because trading volume was similar.
Despite day 3 price is only 5% different from day 2, they earned 8.3x more income, because trading volume was much higher.
It's not the price what matters for their income, it's solely the USD volume that is traded.