Brand name products usually use fancier packaging, they spend money on advertising (direct mail fliers for example), the extra profits encourage more waste in the company since they can afford it, while generic manufactures will go to great lengths to be very efficient. And if money is not spent there it goes to larger marketing budgets, and finally if nothing else to profits, and larger salary. Larger salary usually means larger home, etc, and all that goes with it. So, no, it's not an edge case.
PS. The environment should not be your only reason for choosing or not choosing something to buy. But comparing the price will help you avoid greenwashing.
Without thinking particularly deep about this issue, I suppose your idea runs alongside the efficient market hypothesis. You're making assumptions about the relationship between resource input and product output and perceived value. Like in the EMH, the ideal conditions are perfect information (let's also say perfect money->resource conversion), and rational customers.
EMH advocates would say the system is pretty efficient at pricing already; you could argue that the fancier packaging and expensive marketing takes all the markup into account, so when a customer buys, the products are fairly priced, at least in terms of summing up the steps in the value chain. I'm not so sure about that. Tulips, for example.
So at the cheap end of the scale I would tend to agree with you. For brand markup though, I still think the same rules don't apply as well. It is plausible that the higher the price, the less exact the correlation between environmental impact. In other words, intuition says that "environmental cost of a product is almost exactly equal to it's monetary cost" is less and less applicable as price rises noncompetitively, and brand markup and investor exuberance, are such cases.
Oh, I thought of another example. This is just silly: virtual goods. Anyhow your rule of thumb of buying cheap still holds here (the alternative to buying a virtual good is simply not buying, because you have nothing in both cases!). But this is a clear case where you are purchasing a psychological effect. And I would venture a guess that the more psychology you mix into the pricing the less you can infer its environmental cost.
Alright, I'm stating the obvious... nitpicking over semantics I guess.
Don't forget about profit. The company (i.e. shareholders) are making a larger profit from the marked up item. The money they earn will eventually be spent, and will have an environmental impact.
To your virtual items example:
What determines scarcity and cost? Or more accurately: what is the input into the product to create it?
There are two options here: the items is simply marked up for no reason (i.e. the server releases them rarely for no reason at all), and then the profit situation takes hold.
The other option is that the virtual items are priced correctly, so what makes them rare? (If they were not rare, and priced correctly, then their value is zero.) Usually what makes them rare is the human time spent on making/finding them. Which means that their cost is basically wages, and no different from a real item.
PS. The environment should not be your only reason for choosing or not choosing something to buy. But comparing the price will help you avoid greenwashing.