The S&P 500’s journey to a record high this year has not been inexpensive: By various metrics, stocks have reached unprecedented price levels.

Investors are currently paying more than ever for every dollar of revenue generated by the companies within the index. On Thursday, the benchmark was trading at 3.23 times sales, marking a record high.

While price-to-earnings ratios are not at all-time highs—thanks to substantial profit margins at many of the index’s top companies—they remain at the extreme end of historical values. The S&P 500 is presently trading at 22.5 times its anticipated earnings for the next 12 months, in contrast to the average of 16.8 times since the year 2000.

Many investors believe that the largest U.S. stocks, predominantly technology firms, are worth every cent. Companies such as Nvidia and Microsoft continue to drive sales and profits at an impressive rate, solidifying their market dominance. By the end of July, the 10 largest firms in the S&P 500 represented 39.5% of its total value, the highest ever recorded, as reported by Morningstar. Nine of these companies boast a market capitalization exceeding $1 trillion.

"I’m not particularly concerned about that by itself," stated Steve Sosnick, chief strategist at Interactive Brokers. "The real question is what could occur if the circumstances shift."

In April, investors witnessed the risks associated with the market's reliance on a few high-priced stocks when President Trump’s tariff proposals led to a short-lived selloff. The so-called Magnificent Seven tech stocks underperformed compared to the overall S&P 500, which itself lagged behind if each of the 500 stocks was weighted equally.

"The mix of extremely high valuations and heavily crowded trades certainly increases the market's vulnerability to a prolonged downturn," Sosnick noted. "If everyone is essentially invested in the same assets, where will the new buyers come from when prices drop?"