Get values from Numbers.app into AppleScript's variables
In the process of doing this exercise I understood lists are much better for storing read data. So here is the working code:

tell application "Numbers" to tell the front document to tell the active sheet to tell table 1
    repeat with i from 2 to the count of cells of column "A" -- row 1 is a header
        set theProduct to formatted value of cell i of column "A"
        set theQuamtity to formatted value of cell i of column "B"
        set thePrice to formatted value of cell i of column "C"

        set the end of Products to theProduct
        set the end of Quantities theQuantity
        set the end of Prices to thePrice
    end repeat
end tell