Question

edit for correct pronoun.

we expected the concert to last for at least two hours. since the average ticket sells for twenty dollars, this was not being unrealistic.

Answer 1

To what ONE NOUN does the word "this" refer?

Answer 2

does this refer to ticket or twenty?

Answer 3

If you have to ask, then you need to rephrase the sentence. The word "they" is a vague pronoun and needs to be fixed.

That's how you can find these vague references -- if you cannot name the ONE NOUN to which a pronoun refers, then it is vague, and rephrasing needs to take place.

http://owl.english.purdue.edu/handouts/grammar/g_pronuse.html