The cores/money comparison doesn't looks like it accounts for inflation.
more to the point, the comparison also represents the period of time in which AMD, the one-time "lesser" player nipping at monopolist Intel's heels by competing on price, became the technology leader and started commanding a premium for their products. Meanwhile, Intel has not been forced to cede its position in the market, their existing contracts and business model is based on premium products, not cost leadership. It will take awhile for this to sort out.
Of course Moore's law is slowing down, but cores/$ is an extremely silly metric to use
Does Google Sheets provide a "inflation-adjusted dollar" function?
But anyway, I’m not sure if it makes sense to expect performance/$ to always increase anyway. I mean, I know this started out by talking about multicore, but think about single threaded performance. They’ve already grabbed all the low hanging fruit, the challenge now is finding increasingly hard to hunt down tweaks… a small improvement might require massive engineering effort.
A simple and regular ISA is what makes ARM easier to implement, which also means lower power because of fewer transistors doing thankless work like decoding instructions and reordering them.