Person 1: I have saved $500, and I want to invest it in an ETF.
Person 2: But you will be spending $10, if I am right, as brokerage fee for executing the buy trade. That is a 2% upfront cost for you. And it is big!
Person 1: Uh oh! What should I do?
Person 2: Wait till you can save more, and then buy more units of that ETF in one single trade. This will ensure the brokerage cost, as a percentage of your investment, is lesser, preferably around 0.07%.