2003 Ford Taurus Serpentine belt problem

My Serpentine belt was chirping away last spring..sprayed it with some belt dressing and was fine until around Christmas and it started up again. However, it only seems to chirp/whistle now when it's wet outside. Anyway, I did spray it a few weeks ago and it only helped for maybe 2 weeks? This morning it's whistle seemed a little louder. Does the belt need changing or just more dressing? Also, Is it a costly repair job if it does need changing? I'm afraid it will run me some big bucks in labor.

Any suggestions?
Thanks!!!