And, confusingly, you will hear:
How much is this pair of pants/trousers?
They're £20.
We mix the singular and plural with that piece of clothing a lot. I would say "I have a pair of trousers which are perfect for a wedding". I think many people say "a pair of" but hear the unspoken suggestion of "two". Obviously, we're not talking about two trousers, but "a pair of" with any other noun would mean "two" so the plural verb form comes readily to mind.