Prize winning reds and whites in the grocery store in Portugal and Spain sometimes for under 2 euros (about $2.50). Everything in the grocery store is pretty reasonable actually. We have kept reds in the bilge for several weeks with no ill effects that I can tell, although I am certainly no expert. The bilge can't be that cool either, as the water temperature is close to 80 degrees.
We also have several bottles each of sherry and port after stopping in Porto and Santa Maria at the bodegas. Both seem to be keeping quite well, although they may not get the chance to show how long they can last. Our drinking habits are changing from rum punches to wine, sherry and port.