"It is the duty of a good shepheard to sheer his sheep, not skin them" - Tiberius
Sweet Apple Acres is one of, if not the primary local food producers of Ponyville. They could easily become rich AF by hiking the price up of apples for short term gain. If they did this long term they would ruin the local economy and citizens would move from Ponyville in search of more affordable groceries. So they keep the price low for a steady stream of income to maintain the farm and keep the citizens fed.
Thats why they work so hard during Applebuck Season, The Zap Apple harvest, and Cider Season; that's is when they make meaningful profit.