https://www.amazon.com/gp/product/B01DFKC2SO
Arms and legs are cheap these days!
It's $50 in the USA. So when you consider it's $35 of the raspberry pi, add in $10 for a decent microSD card, at least another $5 for a quality power supply, $8 for the official case, add in some extra there as well for shipping. It quickly gets well above the price of an echo dot when you include all the needed accessories to even reach the same level of functionality.
Like has been said earlier, if you've got all these parts laying around unused from previous projects it's not such a bad deal. To go out and buy all the parts for the specific reason of turning it into an Echo is silly.
Here is an Amazon Smile link to that product.
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But it says right here "#1 Best Seller in Electronics"