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This post is really interesting how much did this all cost I am still confused! on the input into your laptop, mine only has USB, HDMI and VGA and some SDA card slots?

Here you can find all you need to know:
https://www.techinferno.com/index.php?/forums/topic/9658-egpu-experiences-version-20/
Feel free to leave an upvote if I was usefull to you ;)

However to reply quickly to your question the costs that you have to keep in mind are: a gpu of your choice, a psu that can mantain the power needs of the gpu and the riser to do the trick (riser = 50ish €).
And every notebook has a mini-pcie slot, but it's internal, you have to unscrew 1 or 2 screws.