If Bitcoin is at 3$ they get 1.5cent fees. If Bitcoin is at 300$ they get 1.5$ fees.
What does this even mean?
The price of bitcoin doesn't matter. If I want to transfer $100 and bitcoins are trading for $1 each, I need to buy 100 of them. If they're trading for $100 each, I only need to buy one of them. Either way MtGox get the same fees.
The only time it would make a different is if someone wanted a specific number of bitcoins for some reason, but that doesn't happen yet. Because the price is so volatile people tend to still think in fiat currency and convert to bitcoin, rather than pricing things directly in bitcoin.