Actually, "they" does not refer to just "costs." Since we are starting the sentence with a gerund, "they" would have to refer back to the entire phrase "soaring television costs". It's the IDEA of "soaring" that is the subject of the sentence here.

Rephrase: Soaring accounted for more than 1/2 the spending, a greater proportion than it was...

"a greater proportion" refers to the idea of "more than 1/2" so "it" doesn't refer to anything, so we have a pronoun-ambiguity error.

But more seriously, we have a problem with MEANING here. Ask yourself: what is the author logically trying to say?

He's trying to say that "soaring costs" accounted for more, proportionally, in this 1 campaign than in any OTHER campaign. So really he wants to compare TWO CAMPAIGNS. Notice how B nicely makes the comparison clear ("the campaign of 1992...than in any previous election.)

Start by finding the two things that are really being compared.

For your "shares" example, you are comparing the RISE in May with the RISE in June, correct? The best way to word this would be: "The shares rose 20% the value in May than they did in June."