What degree of comparison is used in the sentence: "Some of the groceries are more expensive each week."?
Adjectives that end with -er or have the word 'more' before them are of the Comparative Degree; that is, they compare the person or thing with another of the same type.
In the given sentence, the price of the groceries is compared with the price of the same groceries the previous week; hence the adjective phrase more expensive is in the Comparative Degree.
See this for some details: What degree of comparison is used in the sentence: "Carol needs a quieter office than she now has."...