Here is a picture of the liquid egg whites I use for my frozen egg white dessert each night (you can find that recipe by clicking HERE). I typically just buy whichever brand is the cheapest. But, you really want to make sure you get the 100% egg whites. The "egg substitute" is usually 99% egg whites but has a bunch of other stuff added to it with names I can't pronounce. There will be a spot on the carton that shows you the conversion ratio of liquid egg whites to regular egg whites. I paid $3.99 for the carton and it is the equivalent of about 30 egg whites (13 cents per egg).
Some of you have asked where you can find liquid egg whites from the carton. At most grocery stores they will be located near the regular eggs. Just make sure you get the ones that are 100% egg whites and not the 99% egg whites or egg beaters. The 99% egg whites have a bunch of other stuff added to them (mostly things I can't pronounce) but the 100% egg whites should just be egg whites.