Pricing obtained via Digikey backs up the claims that GDDR6 will not be significantly more expensive than the currently used GDDR5. Of course, these prices need to be taken with a grain of salt, as the cost paid by those who order in large volumes will be significantly lower than the prices quoted below.
PC Games Hardware reports GDDR6 chips with a capacity of 1GB can be obtained via Digikey for 20.78EUR (12Gbps), 21.64EUR (13Gbps) and 22.51EUR (14Gbps).
This compares to 20.01EUR for a 1GB GDDR5 chip clocked at 8Gbps, and 18.88EUR for a 7Gbps version. Basically, the difference between 7Gbps GDDR5 and 14Gbps GDDR6 appears to be just 20 percent at the retailer level, while there's not even a five percent difference between 8Gbps GDDR5 and 12Gbps GDDR6.
How much video card makers pay for these chips is a mystery, but this is an indication that cost will not be a problem for widespread GDDR6 adoption.