The issue - as I see it - can be divided in two.
I mean, if you are really willing to buy an original product, a minimal check can be the price asked for it.
Any good today is made of materials+manufacturing+marketing, it has some costs and a corresponding selling price with a given markup.
Once said that a number of branded items can have a selling price that seem completely disconnected from the relative costs, there are limits.
If you can find an item on the manufacturer site at US$ 100, do you really believe that you can find the SAME item on Amazon for US$ 10?
If you find it at say US$ 80 or 75 or even US$ 50[1] and it comes out as being counterfeited, you may have some valid expectations ruined by Amazon, but if you ordered it for 19.95, free shipping, did you really believe it was "original"?
[1] it is one of my per peeves, if you look at the price of many electronic goods, let's say printers or TV on the official site of the manufacturer they are often double their "street price", either online or at the nearest large shop chain. I often wonder how many printers Brother (say) sells from their site.