Yes, at both ends. Garment cost falls with volume and decoration cost falls because the machine is set up once. The steps are usually noticeable at 10, 25 and 50 pieces.
Volume breaks aren't always linear — sometimes the jump from 24 to 25 pieces crosses a screen-printing colour threshold and saves more than the jump from 40 to 50 does. If you're close to a round number, it's always worth asking whether ordering a few more actually costs less overall.
The questions are free. So is the quote.