Not much, if any. Adding a standard 7-8-inch 4-point foot will increase the weight of a 11/32 cedar arrow about 30 grains, give or take a little. Keep in mind that you don't gain the full weight of the footed material because it replaces the cedar/softwood that was already there so the weight gain is the weight of the footing minus the weight of the cedar that was there in the first place. Regardless, that will increase the FOC about 1%-1 1/2% on most arrows. It will also increase the spine by 1-3#. In short, it's a wash.
I believe you can still get footed shafts from The Footed Shaft in Minnesota. They're a sponsor here. Of course, you can order your footed shafts in the spine and weight you want.