Assuming prices are similar WDW wide, as my experience is with OKW, the prices are higher than a grocery store, but not outrageous, except for the fresh fruit items, like bananas, oranges, etc. Very high for those.
It really depends upon how many groceries you want to purchase and whether they have the brand/item you prefer. We usually drive to WDW so do the bulk of our grocery shoping at Publix, but still pick up some items at the resort.