To be ten a penny means to be very common or ordinary, and in this context Hopkinson is saying that skills are very ordinary.
Consider the statement "Oh, yes, said hopkinson, in the voice of one who considers skills to be very ordinary and commonplace but is well aware of the rarity value of a good bread exhibit.". Is the statement "Oh, yes,said hopkinson, in the voice of one who regards skills as ten a penny but is well aware of the rarity value of a good bread exhibit." false? Yes/no/unknown