Hah. Just about called it...and I was told:6c - £200
8c - £ 300
12c - £ 450
16c - £ 650
my speculation
The 6 core Ryzen 3 has to compete with the i3 so will be priced accordingly. The 8 core Ryzen 5 is completing against i5. The 12 core Ryzen 7 is completing against i7.
The prices you suggest will put the AMD processors well above the price of their Intel counterparts.