That's the thing... Camber doesn't affect your tires nearly as much as toe will, so don't bother with a camber kit if you're in it to save your tires. If you are looking for a larger contact patch with the road, then get one. People that are dropped very low don't even really need a camber kit as long as they pay attention to their tires and rotate them every so often. I need to do that...
When you have negative camber and perfect toe, all the tire is doing is rotating along that one part of the tire. It will wear quicker than normal, but compared to a toe-in situation it is nothing. If your wheels and tires are toe-in, they will not only be rotating in a forward motion, but also inwards. The amount of friction that you put on your tires from toe outweighs camber any day of the week, and friction is what is going to make your tires go. That's what makes your tires grip the road, so if you are essentially dragging them across the ground instead of rolling them, they will get used up much quicker.
Hope that clears things up for you.