Final answer:
The error in the sentence is the use of 'are' instead of 'is' when referring to 'six dollars per pound', which should be treated as a singular amount of money.
Step-by-step explanation:
The error with subject-verb agreement in the sentence provided is the verb 'are' when referring to 'six dollars per pound'.
The subject here is 'six dollars' which is treated as a singular amount of money, not multiple individual dollars. Therefore, the correct verb should be 'is'.
The corrected sentence would be: 'I would rather buy free-range organic chicken,' Marvin admitted to the clerk at All Natural Grocer, 'but six dollars per pound is more money than I can afford.'