Cunard rents by the night. I had to look into it on my cruise because I somehow forgot my tuxedo pants at home. The cost to rent just the pants was going to be $39. I ended up buying a new pair of pants in Naples for less than that.
BTW, if you are looking to pick up an inexpensive tux for gala night I got mine just for the cruise for around $100 at AlainDupetit.com