TORONTO-DOMINION BANK, $73.76, Toronto symbol TD, owns roughly 42% of TD Ameritrade Holding Corp. (Nasdaq symbol AMTD), one of the largest online brokerage firms in the U.S. TD Ameritrade’s chief competitor, Charles Schawb Corp. (New York symbol SCHW), has now eliminated commissions for buying and selling stocks, ETFs and options listed on U.S. or Canadian exchanges. In response, TD Ameritrade matched that policy. Commissions supply about 25% of its revenue, which is why its stock dropped roughly 25%. The shift to zero commissions will hurt TD Bank in two ways—lower income from TD Ameritrade, and drop the value of that holding. However, it’s possible that TD Bank could take advantage of TD Ameritrade’s lower stock price by offering to buy the 58% that it doesn’t already own. That would cost roughly $14 billion U.S., which is small next to the bank’s $134.2 billion (Canadian) market cap. Combining TD Ameritrade and TD’s U.S. banking customers would let the bank cut costs, cross-sell to both client bases…