Imagine that I say that everything costs me a million dollars in value. Maybe there's a sense in which it's true -- I'm just really sensitive. Still, it doesn't seem fair to give me that level of disproportionate influence.

This is a good argument for why it's hard to measure external costs by asking people. It doesn't undermine the definition.

And the situation isn't quite as bleak as you imply, in terms of measurement. We have techniques, such as revealed preference, for measuring costs when people have incentives to lie. We also routinely, in the legal system, apply estimates of "reasonable" costs for things.

The question was, 'how do we measure the intensity of externalities'? We can measure this by peoples' economic choices. The nice thing about money is that it's more objectively quantifiable. You can actually see where money moves. Therefore, the scale, at least is set.

The line of reasoning that this would lead to dumping on the poor relies on the idea that the only way we'd measure economic choices is by adding up the dollar amounts spent. The dollars let us see the choices, but we can be cannier than an adding machine about combining them. At least we can use a disposable income correction, and of course a knowledge correction (if you don't know you have a problem and never end up spending money fixing it - 8Hz noise pollution, say - then we have a bit of a problem with this approach)

What waste products are you thinking of? At the very least, the inside of a carefully constructed toxic waste container won't have any jukes or tree narfs in it.