I was talking about Coppers, not Silvers. Trashing Copper has the (big) upside of making it easier to draw, with the downside of losing some payload. Gaining a Lab, though, is the best of both worlds: you get more draw without losing that $. That's the crucial difference.
Trashing two Estates is like gaining two Labs and taking Miserable. Trashing a Curse is like gaining a Lab and taking a 
token. Trashing a Lab is like gaining a Lab and then trashing two Labs.
This is a decent heuristic but equalizing trashing and drawing is only OK when there is no source of draw/sifting on the board . For example a hypothetical card which says
+3 Cards +1 Action | When you gain this, gain 2 Curses. seems, if one uses such static, non-comparative analysis, like getting a cantrip and taking Miserable, i.e. not something you ever want. But perhaps there are sifters or trashers that can deal with the Curses such that the Double Lab is actually a pretty strong card.
You always have to analyze comparitively, a board can have drawers, sifters and trashers and the relative strength of each influences the relative strength of the other cards. If there is only Trade Route to deal with the junk, Trade Route is stronger than in a situation in which there is also Dungeon.
You always have to analyze dynamically, i.e. when you use e.g. Hideout as a trasher and a village you might want to trash slower than in a situation in which you only use it as trasher. Tempo always matters and it influences how quickly you want to trash, how quickly you want to build up draw power and how quickly you want to build up payload.
These intricacies are, at least to me, at the core of the game. If things were as simple as
Cathedral = 3 Labs, easily by far the most lunatic thing I ever heard concerning Dominion, the game would be solved and not be as opaque and and as interesting as it is.
