There is absolutely no doubt that the 7900 series are more expensive to manufacture per performance than any of the 40 series, except the 4090.
The 40 series is truly brilliant engineering, wringing the guts out of tiny memory busses and memory pool, trivially simple PCBs, power designs etc. The issue we have with it is the pricing, which is woeful compared to BOM cost vs. Prior gens.
The 79 series is not optimal, due to the overhead of all the IOs, silicon interposer, extra processes for chiplets. However, this approach does render some decent benefits, that will play a much larger role in future designs. The ability to have cheaper nodes for cache and IO, and denser nodes for compute makes a lot of sense. You might even have separate dies for AI compute vs. "standard" GPU compute.
TLDR: 79 series is expensive to manufacture vs. 40 series for the same performance. Future chiplet arch's will scale well and should have better price to manufacture vs. Performance