Here’s a short test I did a year ago. I rarely make such comparisons, so don’t take the images below too seriously.
Voigtlander 58mm 1.4 (V), Sony 50mm 1.8 (S) and Sony Zeiss 55mm 1.8 (Z), tested on a NEX-7.
Looking at the Zeiss and its glowing reviews, something is wrong. Maybe I have a bad copy, maybe the NEX-7 sensor is being picky, maybe I messed up the focus (probably the latter, as I am actually very happy with the lens). Still, is the FE55 worth twice as much based on what you see here? Even if there’s an issue with the test, how much sharper would one need their lens to be when looking at the Voigtlander and the SEL50F18 (and how much sharper the FE55 can be)?
I omitted F5.6 to save some space, as there isn't much difference between F4.0 and F8.0.
It is a good idea to put things into perspective occasionally. As it is the norm today, there are plenty of measurements, 100% crops and overblown statements. Lenses are primarily reviewed and compared based on their sharpness and sharpness has undeservedly risen to an unprecedented level of relevance (mostly thanks to modern high-resolution sensors). It doesn’t take long to find a thread explaining how a lens is “crap”, because it is less sharp than another one worth three times more and weighing twice as much.
Lens “performance” is actually a very stretchy concept, depending on lens, camera, output, personal preferences and budget. What good is sharpness if there’s a huge colorful blob in the middle of your image? Or, if the lens is so big or expensive that you hesitate to take it with you? Suddenly, it all becomes very relative.
Now, let's look at a couple of more realistic scenarios.
The above was shot with a Nikkor 135mm F3.5. For an almost 40-year-old lens, it works quite well (well, I don't know of a really bad 135mm lens, but still). This is my weakest lens in terms of sharpness, but I wouldn't have any issue printing quite large with it (after some proper sharpening) and I often crop heavily. It was cheap even at its release and I paid around 2000 yen for mine. So, if we look exclusively at sharpness, it would make most modern lenses look quite bad in terms of value. While something like the new Zeiss 135mm Apo will be better in every aspect of image quality (not to mention being faster), it certainly isn't 100 times better. Hence, the diminishing returns.