OP does ask specifically for purchasing power, but it is sometimes overlooked that wages have generally inflated faster than goods prices have. In other words, a larger share of wages had to be spent on subsistence food in older times. Obviously this would affect the perceived value of a penny: even if it only bought a bun, that bun would represent more wealth to a laborer.
Variations by place, occupation, hours-per-week, etc. may make comparisons difficult, but here’s a page that shows wages in England between 1264 and 1980.
Well in 1893 My great Grand Father bought the ranch 120 acres with livestock and farming equipment for “the sum of $10.00 US gold coin”. I sold 20 acres of the ranch in 2005 for $350,000.
I suspect the disconnect between the dry inflationary decrease in value and the presumption that a penny was actually worth something back then is due to the fact that food commodities hold down the CPI-based inflation rate. Wheat is now about $9 a bushel, versus $1.02 in 1870; sugar is $0.19 per pound today, versus $0.07 then. Stick butter is $3.23 per pound today, versus $0.15 then. So food prices have gone up by as little as two and a half times, and 21 times at most. It’s worth remembering that wholesale prices for agricultural goods actually fell significantly from 1870-1900.
Conversely, gold is currently $1,650ish per troy ounce, versus $22.88 in 1870, or just over 72 times more. GDP, adjusted for PPP, is now $15 trillion, versus $99 billion, an increase by a factor of 152.