Learn German: Die Idee ist gut, aber sie ist teuer. - The idea is good, but it is expensive.

The verb is actually in the second position! The word aber (but) is a special "Position 0" conjunction. It acts as a bridge between the two sentences and doesn't count towards the word order. Therefore, the pronoun sie takes position 1, and the verb ist takes its normal place in position 2.

Why does the sentence use sie (she) instead of es (it) for "it is expensive"?

In German, pronouns must match the grammatical gender of the noun they replace. Because the word for idea is feminine (die Idee), you have to use the feminine pronoun sie to refer back to it. English uses "it" for all objects, but German always looks at the "der/die/das" of the original noun!

Is the comma before aber required?

Yes. When aber connects two complete mini-sentences that each have their own subject and verb (like Die Idee ist gut and sie ist teuer), German grammar requires a comma before the conjunction.

Could I shorten this and just say aber teuer instead of aber sie ist teuer?

Absolutely! Just like in English, if the subject and the verb are exactly the same in both halves of the sentence, you can drop them in the second half. Saying Die Idee ist gut, aber teuer is perfectly natural German.