I am confused!

When i express i only get 20-40 ml breastmilk while replacing a feed but when i feed directly my baby seems satisfied. He generally takes 4oz when feeding from bottle. So I am super confused why i am not able to pump more, it makes me doubt my supply. This is the reason i was giving him formula after everyfeed but from past 20 days i have only given him formula once a day coz he seemed satisfied after breastfeeding. I have used philips avent electric breastpump also medela hand pump. I don’t think i can buy more breastpumps as they are expensive. I don’t know what to do.