assume the period is one year:
the borrowing cost is 1,000,000*1.06=1,060,000 USD
the spot rate is 0.6440/50DM/USD, so the principal equals: 1,000,000*0.6440=644,000 DM
the holding benefit of DM for one year is: 644,000*1.1=708,400 DM
Then convert it into USD equals:708,400*1.85=1,310,540 USD
the arbitrage income is 1,310,540-1,060,000=250,540 USD