Nは 一つ いくらですか
This pattern is used to ask the unit price (e.g., 一つ、一本、一ぴき、一さつ) of something.
ボールペンは 一本 いくらですか。 How much are the ball-point pens each?/How much is one ball-point pen?
Particles are not used after words which expresses quantity/amount.
