If you are looking for lower prices as far as grocery shopping in Kissimmee goes, it has been my experience when vacationing there that the Publix grocery stores do offer cheaper prices for food items.
On the other end of the scale, I have noticed that Goodings tended to be quite a bit higher in comparison as far as food selections are priced.
Another great alternative in this area for discounted grocery shopping (besides Publix) would be the Super Wal-Mart store located in the area.
Hope this helps.