First, that's odd you say this because I just punched in a random route that I knew Spirit would fly into priceline to see what it would return, and viola, there's a link right below the itinerary that says "Baggage Fees". You click it and a window will pop up with what the baggage fees are (as well as carryon since Spirit charges for carryon baggage that can't fit under the seat in front of you and must go into the overhead bin). My experience with online travel agencies is somewhat limited, but that seems to be pretty well laid out IMO.

Second, what does this have to do with DL that you posted this in the Delta Skymiles forum?