Wheel Brightener is cheap as dirt, and it dilutes 4:1. I think it works out to something like $5 a gallon for the final dilution. S&R is also good, it's a little safer to use/have around, but at 6 or 8 times the price (30 or 40/gallon I can't remember which), I'm a Meg's man.
Amazing Roll off and DP S&R aren't even in the same category as these 2. Those two are alkaline cleaners, which I've never been impressed with. S&R and WB (much more so WB) are acid-based, and therefore MUCH more effective at breaking the chemical bonds between brake dust and metal.
