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The reason why laptops with user replaceable GPUs are expensive is because only higher end mobile GPUs are manufactured in the MXM format. MXM is basically like a mini PCIe card for laptops. Including such an interface in a laptop increases the cost to manufacture the laptop. Putting a mobile GPU on a MXM card also adds more costs. That is why MXM GPUs are limited to higher end GPU chips like the GTX 1060.
MXM GPUs are not sold in the retail market. You basically need to go to a site like ebay to purchase these GPUs. Here's a MXM format GTX 1060 which sells for $730.
https
/www.ebay.com/itm/Nvidia-GTX1060-MXM-Video-Graphics-GPU-Card-N17E-G1-A1-6Gb-gddr-5-/263522971034
However, older MXM format GPUs are a bit cheaper like the following GTX 980m for $468.
https
/www.ebay.com/p/NVIDIA-Gtx980m-8g-MXM-SLI-N16e-gx-a1-Video-Graphics-Card-for-Notebook-Laptop/2096621532?iid=222851932999
If you are on a limited budget, then do not bother with laptops that have a MXM interface port.