GDDR achieves higher speeds than normal DDR mainly by specifying much tighter tolerances on the electrical interface, and using wider interface to the memory chips. This means that using commodity GDDR (which is the only fast DRAM that will be reasonably cheap), you have fairly strict limitations on the maximum amount of RAM your can use with the same GPUs that are manufactured for consumer use. (Typically, at most 4x difference between the lowest-end reasonable configuration and the highest-end one, 2x from higher density modules and 2x from using clamshell memory configuration, although often you only have one type of module for a new memory interface generation.)
If the product requires either a new memory or GPU die configuration, it's cost will be very high.
The only type of memory that can support very different VRAM sizes for an efficiently utilized bus of the same size is HBM, and so far that is limited to the very high end.