I don't have a number for Hess, but I can tell you that we did some of our shopping there last June.
We used the one that is next to the Epcot resorts. The prices there were comparable to grocery stores in the area. They had soft drinks, water, milk, bread, junk food, beer...you know, regular convenience store stuff.
While you can't do extensive shopping there, we found it to be just right for picking up extra drinks, snacks, etc. We're not the kind of people who want to fight crowds shopping at the bigger stores while on vacation, so the Hess station worked out fine for us.
If you do need more groceries than you can find at Hess, I recommend going out to Lake Buena Vista. They have an excellent Goodings and they did have a Winn Dixie out there too. A little higher than the superstores, but never crowded. You can get in and out very quickly.
I don't know which resort you're staying in, but this Hess station is right across the road from the Epcot resorts. We were at the YC and it was a 2 minute drive over there.