It depends if you're talking about: a) RDNA2 versus RDNA, or b) different capacities within the RDNA2 product stack.
The Infinity Cache scales with the die size (compute units) so it's essentially impossible to directly quantify. You can't isolate two products where the only variable is cache size. And even if it does make a difference, how would that influence purchasing decisions? Again it's hard to say because there's no situation where cache size is the main difference between two products.
AMD have some slides comparing the generational improvement percentages which mention the Infinity Cache. But that's about it.