You can't replace daar with er in this sentence. Er is used when there refers to something said earlier. For instance: Zie ik je morgen bij mij thuis? (Will I see you tomorrow at my place?). Ok, maar hoe kom je daar/er (Ok, but how do you get there?).
No, you cannot replace either 'hier' or 'daar' with 'er' in this sentence. Because a comparison is made ("here" vs "there), there is a natural emphasis on these words, hence 'er' (the unstressed form) cannot be used. :)