It's very little due to "greedy retailers", and more due to the realities of operating in smaller, less unified markets. The US has massive economics of scale due to being a 300m people, wealthy, single-language market, meaning tons of sales of high margin products with relatively low staff needs and increased opportunity for centralization of various branches of the retail value chain. In contrast, while the EU is nominally a single market, it is still fragmented through language, distribution chains, localized economic factors, and more. This drives up prices simply because there are more people involved, more business costs that need covering in order to deliver the products onto retail shelves.
Germany being the cheapest in Europe just confirms this: it's the largest single market, and has a relatively high level of income, meaning it gets a lot of the same advantages as the US, just to a lesser degree due to less scale. The only major difference would be that many brands have direct distribution in the US (meaning AMD, Nvidia, Asus, and others sell directly to retailers rather than to distributors), which is again a limitation of market size - running your own distribution business is expensive and difficult.
After nearly a decade in electronics retail, even if that's a while back now, I can verify with 100% security that for most retailers, margins (especially on expensive products like GPUs and CPUs) are razor-thin, often below what is actually needed for the company to break even, let alone make any net profit.
