The price you pay matters, but surprisingly, sometimes it’s hard to know what that price is when you’re talking about a Vanguard ETF.

Vanguard’s Advisor blog recently published an article looking at the cost of holding a Vanguard ETF versus other popular ETFs—see here. It’s a decent piece with some good graphics. (Though Vanguard has mislabeled the second graphic in the article—the title of the second chart says “Total cost of ownership: VOO vs. SPY” when it should say “VCIT vs. LQD.”)

The short story is that while trading a Vanguard ETF might be more costly due to larger bid-ask spreads, the lower expense ratio wins over time. (A bid-ask spread is the difference between the highest price a buyer is willing to pay and the lowest price a seller is ready to take.)

No arguments there. But also, the argument quibbles over basis points—hundredths of one percent.

For example, Vanguard compares its S&P 500 ETF (VOO) with an expense ratio of 0.03% to the SPDR S&P 500 ETF (SPY) and its 0.0945% expense ratio. Even over a decade, according to Vanguard’s math, we’re talking about a difference of $600 or so on a $100,000 investment (or 0.6%). That’s not a life-changing amount of money, in my book.

Exchange a fund with an expense ratio of 1% or more for one charging less than 0.10%? Now we’re talking.

Yes, you should always consider cheaper funds. But recognize that picking between similar funds charging 0.03% and 0.09% isn’t going to make or break your investment results. You’re already at rock bottom. (In fact, in August, State Street announced that its “SPDR Portfolio” S&P 500 ETF (SPLG) will charge just 0.02% from now on.)

This splitting of hairs (or basis points) is made all the more ridiculous given the variability of ETF performance reporting.

Mutual funds are easy. They always trade at NAV. Everyone receives the same price when they buy or sell shares on a given day. All investors reinvest distributions at the same price, too. On top of that, the prices are clear and transparent.

ETFs are almost the exact opposite.

Go to Vanguard’s website, and you’ll see performance reported several ways.

Vanguard reports returns based on an ETF’s NAV. That’s helpful but irrelevant since no one can buy or sell an ETF at its NAV with any reliability.

Vanguard also reports a “market price” return. But they define market price as the midpoint between the bid and the ask. Sorry, but no one trades at the midpoint. It’s a made-up price.

On top of that, there’s the question of what is used when reinvesting distributions back into your ETF. Is it the closing NAV? Market price? Halfway between the bid and the ask? Or something else?

In my analysis and reporting to you, I strive to use ETF returns based on those of an actual investor. Between me and the guy I share an office with, we own dozens of Vanguard’s ETFs. The ETF returns I report are based on the reinvestment prices we receive in our Vanguard accounts. Not Schwab accounts. Not Fidelity accounts. Vanguard brokerage accounts. Sounds simple, right?

Well, this month, figuring out those reinvestment prices has been harder than usual. I’ll explain.

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