That is proper and natural in speech. English isn't arithmetic. The plural way would also work, "The price indexes for corn and wheat were 10 and 50, respectively," and would be better in formal writing.
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englishnewbieThe price index for corn and wheat was 10 and 50, respectively. Can I use "index" to refer to 10 and 50 like this? Or does the subject have to be a plural?Yes, it does.