The formal shops where I live sell used tuxes for about $150. I may buy one before my next cruise--renting one cost almost as much.
Nice tux shirts often show up at Marshalls and other discount stores, or at outlet stores (like Armanis outlets).
On my first (Caronia) cruise, my wife and I were staying in England for the week after the cruise, and since it was prime tux-renting season (late May), the local stores would not rent a tux to me. We called Cunard, and they said I could rent a tux on board, so I reserved one in my size. Then, the night before we left to fly over to London, Cunard called (from England) to say that there would be no tux rental on the ship. In a panic, I went to Lord & Taylor that night and bought a black suit, and the seamstress hemmed the pants while I waited.
When we boarded two days later, Caronia had tuxes available for rental, including a full selection in my size!
I've never quite understood how all this misinformation occurred, but suffice it say that I have a black suit in my closet that doesn't get much wearin'!