Sure you could say that people's experience of something like sweetness is subjective, but you could say the same thing about spiciness.
Yes, and Wikipedia has a brief summary of these scales (with some further details in other portions of the article and the links).
Basically, at least four of the five recognized primary "tastes" have a reference compound that other foods are compared to subjectively. For sweetness, a solution containing the test compound is diluted until sweetness can barely be detected by a human taster (similar to the Scoville scale). Sucrose is given the reference value of 1 (or sometimes 100). Sourness is similarly rated in comparison to a dilute hydrochloric acid solution, saltiness is rated in comparison to a dilute table salt (sodium chloride) solution, and bitterness is rated relative to a dilute quinine solution. (I've not heard of any similar scale for umami.)
In most cases, the reference compound is given a value of 1. The most common way to reference these scales is as the "[taste] index," as in "Citric acid has a value of 0.46 on the sourness index," which means it has to be diluted slightly less than half of how much hydrochloric acid would be for it to be on the threshold of human sourness detection.
(By the way, while these scales show us something, they are mostly useful for comparing single pure substances. For culinary purposes, their value is somewhat limited by the complex interactions among various tastes and flavor components. Even individual substances can change flavor depending on environment: for example, a complex molecule may taste relatively neutral at neutral pH, but with increased acid and thus sourness, it may acquire a salty flavor.)