In the world of exchange-traded funds (ETFs), it is no secret that low fees attract investors. A slew of data points confirm as much. Among this year's top 10 asset-gathering ETFs, only the iShares MSCI EAFE ETF (EFA), which tracks the popular MSCI EAFE Index, can be considered remotely pricey by today's standards.

EFA charges 0.33% per year, or $33 on a $10,000 investment, but that has not prevented the ETF from adding $9.48 billion in new assets this year, a total surpassed by just six other ETFs. Overall, 0.33% is not all that expensive in ETF land, but when it comes to expense ratios, there is an area where investors are showing in a big way that they do not want to drift past. (See also: Pay Attention to Your Fund's Expense Ratio.)

"Price competition has intensified in the ETF business, especially among broad-based vanilla funds. These days, the cost line in the sand is around 0.20% per year, with the biggest gains going to funds that cost 0.05% per year or less," according to FactSet.

September served as a microcosm of investors' preference for low-fee ETFs. Those funds with annual fees of 0.05% or less hauled in $3.46 billion in new assets last month, while ETFs with fees of more than 0.2% ($20 per year on a $10,000 investment) lost $3.13 billion in assets in September, according to FactSet data. Among this year's top 10 asset-gathering ETFs, only the iShares Core S&P 500 ETF (IVV), Vanguard S&P 500 ETF (VOO) and iShares Core U.S. Aggregate Bond ETF (AGG) have expense ratios of 0.05% or less. However, plenty of other members of that list are not far above the 0.05% mark. (See also: 3 Ways to Avoid High ETF Fees.)

As has been widely documented, the iShares Core MSCI EAFE ETF (IEFA), the low-cost answer to the aforementioned EFA, is on a torrid pace of adding new assets this year, a theme that continued in September. "In September, vanilla ETFs offering exposure to developed markets outside the EFA had no inflows at all, while its in-house IEFA swept in $2 billion," said FactSet. "Same issuer, similar underlying index, but at a fraction of the cost. Why pay 0.33% when you can get the same portfolio, with additional small-cap exposure, for 0.08%?"

Year to date, IEFA has taken in almost $17.6 billion in new assets, a total exceeded by just one other ETF: IVV. Six of this year's most prolific asset-gathering ETFs have expense ratios of 0.07% to 0.15%. (See also: This ETF Just Keeps Growing and Growing.)

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