There is only so far you can go with those sorts of comparisons. For instance, a lvl 2 city played early on gives you a 6 card hand which is good odds (with a silver opening) to hit the magical 6 coin break point. A 4 card hand will have a strong shot at 5 coin, but if the 5's are weak, worthless, or simply not there then the differential is MUCH smaller than an early lab. Likewise, being able to play two terminals (due to synergy) than not being able to play one. For instance a free lvl 2 city makes all sorts of things like militia -> young witch or IW (gain a tunnel) -> young witch possible.
Pretty much early on, the hand size differentials are more favorable for the bishop than the "free lab". Later, the odds of them having a desired card to trash are lower while the odds that they could use synergistic actions are much higher.
Really, early bishop is all about the opportunity cost. What could you buy instead of bishop, how much will that help you compared to bishoping sooner? There are some niche situations where your bishop is heavily countered (opponent having strong counters like menage, market square, feodum, etc.) or where the opponent gets a lot less from trashing (e.g. opening baron, coppersmith, apothecary, etc.), but most of the time the other guy doesn't get enough benefit to make up for the chips so really are just down buying something else (which is a VERY non-trivial penalty).
