It’s 34 years, and I have been playing these games for a very long time.
You’re right that it is basically impossible to compare values precisely between different economies, because the price level is a vector, and in the case when the economies supply very different commodities they are vectors in different vector spaces. My iPhone is a combination of things that would have been expensive in 1985 (portable phone, camera, video camera, compass, e-mail gadget, clock, diary, library, calculator, flashlight, TV receiver, street directory etc. etc.), but also of some things that I could not then get at any price (private radio station, streaming video viewer, locator/navigator). This makes the standard currencies of generic RPGs such as GURPS and ForeSight a hopeless project.
The best you can hope for is the kind of thing that I am trying to do here — convey an impression of how expensive a thing is to people in the reference economy. Labour ought to be the universal numeraire, but the reality is that an hour’s minimum wage in modern Australia is both a lot more value and a lot less important to a worker than an hour’s wage of an unskilled labour in a mediaeval-tech hell-hole where the labour supply is equilibrated by starvation.
So for individual expenses I think that I am doing as well as I can to say that an SVU (an Imperial crown in Flat Black, an obol in Gehennum) is about what you might pay for an eat-in cooked lunch on a workday when you were concerned with assuaging your hunger and not with gourmetry. That gives as good an impression of value as anything, and escapes most of the dangers of specifying a particular commodity. The lunch might be a burger and fries with a coke, or a plate of chow mein with tea, but “the price of a cheap eat-in cooked lunch” is the best we can do.
And when it comes to building an emperor’s palace on Thekla Bay or the capital of an interstellar polity with a trillion population, the significance of the cost is its relation to GDP. I try to do that all in minai and crowns, but some point of reference for the early-2st-century WEIRD roleplayer is required.
As @MichaelCule doubtless recalls, price is an illusion, the price of lunch doubly so.