The minimums are usually the same as they are for having a reception in the location you choose, so you could use the pricing info for the various wedding reception locations as a starting point. People have also used Sea Breeze Point for catered dinners - I believe the food and beverage minimum is $1,000 or $1,200. You could also do a beach-side reception at Shipwreck Beach at Yacht & Beach Club or possibly Clementine Beach at Fort Wilderness, although I've heard they aren't using it anymore.
The Summerhouse, Marina, and Sago Cay Pointe at the Grand Floridian are also popular locations. The Summerhouse is a bar area adjacent to the beach, about halfway between the resorts theme pool and courtyard pool. It has a food and beverage minimum of $1,500. There is no venue rental fee. The food and beverage minimum for the Grand Floridian Marina is $1,500, and it's $500 for Sago Cay Pointe. The Marina requires a minimum of 20 guests and has a maximum of 120, while Sago Cay Pointe accommodates 1075 guests. Combined, the sites hold up to 300 people. There are no venue rental fees for either site.
Your planner can help you figure out someplace that meets your budget and your group size.