Acutally it's only 77% protien by volume...you have to have two scoops to get to the 50g mark. Optimum nutrition 100% whey is about $50 for a 10lb bag and is 78% protien by volume. I've actually been using some stuff that is over 90% protien by volume from Bioplex called pure WPI. It has 0g carbs, 0g of sugar, 0g fat, 0g Cholesterol... Is about the best I've found. AST VP2 would come in second...at 85% protien by volume...
The bioplex stuff is a bit spendy...about $50 for 5lbs at my local sup shop...haven't looked for it online yet though...since it is a higher concentration it is actually cheaper than the Isopure stuff since 10lbs of it is actually 9lbs of protien versus 7.7lbs of Isopure - both Isopure and Pure WPI are about the same price by volume though...