Commodities are trading at the lowest prices relative to U.S. stocks in half a century or more as a bear market persists. The ratio between the S&P/GSCI Total Return Index and the S&P 500 Index dropped Monday to its lowest level since calculations of the commodity gauge begin in 1970, according to data compiled by Bloomberg. Since peaking in July 2008, the ratio has plunged as much as 91%, including a 6.2% decline for the year through Monday. This year’s slump follows losses in 10 of the past 11 years. The exception was 2016, when the S&P/GSCI — based on 24 commodity futures — rose 1.7% in relative terms.