How about the idea of creating a new product - a modular graphics card with replaceable and expandable memory of different performance / timings? Or even with a removable processor.
This allows users to purchase your card at the lowest price and subsequently increase its performance as far as financial possibilities are concerned.
It is more cost effective to make a GPU they way they are made now. Also you cant buy a "cheap" card and upgrade it down the road. Look at a mainstream GPU like RX 480. Its $240 MSRP for 8GB version. Now look at the most budged R3 + B350 + 8GB of RAM. Its around the same price so you do not save anything.