well the usual amount i see around 5-6-7 egg whites. maybe 1-2 whole eggs. and how cant you afford eggs? :\ 99cents for 30 whole eggs here. if your getting the seperated egg whites, its cheaper to just boil up whole eggs and eat the egg white.
If you're just eating the whites, it's a lot easier to buy egg whites in cartons, pre-separated. They're basically protein, so feel free to eat a bunch! I think the usual serving size, though, is about 1/2 a cup.