That adage applies to no-load mutual funds. If you're one of those investors, you may want to compare the cost to buy versus the cost to own.
Every mutual fund has annual expenses.
The most obvious are fees paid to the fund's investment advisor, as well as marketing, advertising and distribution charges. Other fees include normal operating costs, such as office space, staff and equipment. Such expenses are not charged directly to a shareholder's account, but are deducted from the fund's assets. This means that there is less money for shareholder returns. Lower expenses result in more money for shareholders over the long term.