Recently, at seekingalpha.com, I spotted an interesting little piece on exchange-traded funds (ETFs) -- which combine mutual funds' diversification with stocks' convenient trading -- and their revenue per fund. The author, Jim Wiandt, was wowed to discover that the ETF industry is taking in some $2 billion in fees annually.

Wiandt also listed the top 50 ETFs according to their revenue, which he arrived at by multiplying their expense ratio (essentially the annual fee you'd pay to hold the ETF) by their total assets. For example, an ETF with an expense ratio of 0.90% and $1 billion in assets would have $9,000,000 in revenue.

Here are the top-grossing ETFs, according to Wiandt's figures:

Fund

Expense Ratio

Assets

Revenue

iShares MSCI Emerging Markets (NYSE: EEM)

0.74%

$26.0 billion

$192.6 million

iShares MSCI EAFE (NYSE: EFA)

0.35%

$46.9 billion

$164 million

streetTRACKS Gold Shares (NYSE: GLD)

0.40%

$20.0 billion

$79.9 million

SPDRs (AMEX: SPY)

0.10%

$66.5 billion

$66.5 million

iShares Brazil (NYSE: EWZ)

0.74%

$7.5 billion

$55.7 million

iShares Japan (NYSE: EWJ)

0.57%

$8.2 billion

$46.5 million

iShares FTSE/Xinhua China (NYSE: FXI)

0.74%

$6.2 billion

$45.6 million

Source: Seekingalpha.com

I hadn't realized that the iShares ETF family was raking in so much money. The iShares MSCI Emerging Markets ETF alone collected nearly a fifth of a billion dollars. By instinct, I prepared to get outraged.

Then I stopped and thought about it for a minute. Sure, no investor likes to get fleeced. But are investors in such ETFs being poorly served? Shareholders of the iShares MSCI Emerging Markets ETF have earned an annual average of 29% over the past three years -- not too shabby! Those in the iShares FTSE/Xinhua China fund, meanwhile, have earned an annual average gain of 40% over the same period.

Paying for performance
What matters most is how well shareholders do. If a lavishly compensated CEO is delivering the goods, significantly enriching shareholders, it's harder to complain than when the company is sinking and the CEO is getting big raises. The same goes for mutual funds and ETFs.

Still, it's worth examining expense ratios when looking at ETFs and mutual funds. They will affect your ultimate profit, after all. You usually have a choice between great ETFs with low expense ratios and great ETFs with high ones. Learn more here.

Longtime Fool contributor Selena Maranjian owns shares of an S&P 500 index fund. Try our investing services free for 30 days. The Motley Fool is Fools writing for Fools.