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.
To what ONE NOUN does the word "this" refer?
does this refer to ticket or twenty?
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