How much does it really cost to invest in mutual funds? While things like expense ratios are intended to give a rough idea, legendary Vanguard Founder Jack Bogle is attempting to take a deeper, more comprehensive look at the issue in a yet-to-be-published paper.

The paper, reports Morningstar’s John Rekenthaler, will be published in the Financial Analysts Journal and offers the first examination of “the six elements of mutual fund costs … in a single table. They are sorted into two bins: costs that are inextricably attached to a fund, so that they cannot be avoided by any shareholder; and costs that vary by investor.” The fund costs include the expense ratio, transaction costs, and “cash drag” (which assesses how much investors lose when a fund keeps a chunk of their money in cash rather than investing it); the investor costs include sales charges, tax inefficiency, and behavioral factors (buying high and selling low).

The final tally: For the fund costs, the average is 1.77%, according to Bogle’s research, Rekenthaler says. The investor costs are another 2.15%, making for a total of nearly 4% in costs.

Rekenthaler looks at how Bogle arrived at his estimates, the pros and cons of his approach, and the broader significance. “Fund costs require few adjustments,” he says. “The figure lines up nicely with observed performance. … Investor costs are more difficult to interpret. They depend highly on the situation. They also do not lend themselves to a simple distinction between actively managed funds and index funds. A buy-and-hold investor purchasing directly from a fund company for a tax-sheltered account will have low investor costs, even if using actively managed funds. In contrast, those who like to trade, who pay for investment advice, and who are using a taxable account will have higher investor costs. That is so even if they own index funds. The concept of investor costs is very useful, however, as is Bogle’s attempt to quantify those amounts.”