Your examples have an interesting pattern.
> How much was a dose of penicillin, an iphone or a flight to New York in 1819? There's a very real sense in which very average Westerners now are far wealthier than Darcy.
One's wealthiness is always relative to others. The items listed here are goods that are broadly available today across wealth bands, and are therefore not markers of wealthiness.
> Conversely Darcy would have had far more power than the someone with net worth £16.8-28m would today. He'd probably have had a large say in who the local MP was, the local priest. Probably could turf a large chunk of the local population out of jobs and homes. Likely had a small army of servants.
Power is a great (the ultimate ?) example of a resource whose access and distribution can be compared over time.
Power over others has never been quantified like monetary wealth has, yet it is intrinsic to humans given we are a highly social species. For people like Darcy, such power over others was a hereditary birthright. You couldn't buy a share of that power as an investment. Therefore, there was no market that could estimate the monetary value of that power.