In case anyone believes this comment (that "until recently" there have even no price drops), I will throw in an alternative anecdote (as I'm at a restaurant and am not interested anyway in digging up alternative evidence: readers can do that themselves), Amazon has decreased their prices for all kinds of things many times; bandwidth quite often, servers a few times... I actually find myself misquoting Amazon's prices in conversations often due to having lost track of random price drops.
The one thing that hasn't dropped is where 99% of my costs are: the price of a PUT to S3.
We're talking about two different things -- price drops vs. discounts.
They drop prices across the board all the time. They don't do individual negotiated prices at time T which are different for one customer vs. another customer. (they just started doing this a few months ago, in the form of offering special services to some customers but not others, and now in terms of volume pricing discounts).
All hosting providers drop prices over time (mainly by increasing the amount of RAM per server at the same price, vs. dropping the pricing of packages) for some things (and tend to raise for power, cooling, and physical space). But everyone except Amazon usually also does individual negotiation of prices.
Amazon do negotiate individual prices in some circumstances.
I'm not entirely sure what circumstances it takes to make them negotiate, but the discounts I've heard about are significant. It doesn't seem to be based on the size of the deal though.
The only times I've heard of this were GovCloud (which costs more, but includes a lot of extra service) and the recent announcement of volume based pricing (which is sort of individual in a way).
Dropbox, Heroku, and Netflix would be the authoritative data points, I think. I've heard "won't do anything custom", but I never really pushed to find out about discounts. Even if they do exist, they're nowhere near as common as with every other provider (where either you get a marketing deal discount, or can just get a discount for the asking, especially early on, or as you grow).
The one thing that hasn't dropped is where 99% of my costs are: the price of a PUT to S3.