Knowledge is expensive. But even more so, is stupidity.
Knowledge is expensive. But, stupidity is even more so.
I'm very confused as to the correct punctuation and order or the above phrase.
For the second, I think you should use:
It is probably the better 'Use of English'.
If I was punctuating the first, I'd probably use a very fussy style:
The last is least fussy, and most in the modern style of minimizing commas, but it still doesn't feel comfortable; the second sentence ('Knowledge is expensive, but stupidity is even more so') is better, I submit.
Finally, isn't it 'ignorance' rather than 'stupidity' that is more expensive than knowledge?
Quoting the OP
The second one is just wrong.
You could always go with
You're chaining two related statements together using the word but.
You could also use it that way without the semicolon, using a period instead. Otherwise, I would just try and leave out the "but".
A lot of punctuation, and especially commas, I think you can determine just by speaking the sentence out loud.
If you speak "Knowledge is expensive. But stupidity is even more so", do you pause (more precisely, end an intonation group) after "but"?
Well, you might do so, for rhetorical effect, if you were making a speech: "But ... stupidity is even more so". But in normal speech you would not separate "but" from "stupidity", so you don't put punctuation in.
Your first example, "Knowledge is expensive. But even more so, is stupidity", is an example of chiasmus, and again would be more appropriate when making a speech than in ordinary conversation. In that rhetorical context you might pause after "even more so", but I think that in a more normal delivery you would not, so a comma is not appropriate. But I think the case is less clear than with your second example.
The better order is
Both orders are grammatically correct, but this one puts the important word "stupidity" at the end, where it gets emphasis.