Say you have Fund A and Fund B set to automatically invest 50/50 your $100 dollar contribution bi-weekly, with a buy commission of $5. You pay buy commissions on fund A and B 4 total times a month, so you've wasted 10% of your monthly investment ability ($200 - $20). In a year that money is $194 at 8%. (This used to be the case with old ShareBuilder/ING/Capital One---not sure about the new-style other brokers)

Consider a monthly payment to Fund A and two weeks later a monthly payment to Fund B. You saved half commission cost of above, and in a year your return is ~$205.