2002 Cadillac Deville Rear main oil seal

Engine problem
2002 Cadillac Deville V8 Front Wheel Drive 103000 miles

I have a 2002 Cadillac Deville that is in excellent shape. Interior and exterior extremely clean, runs good, and rides well. Over the past several months I have had a slight oil leak from the rear main seal that has gotten progressively worse. The car uses some oil (understand that most Cad this model do) but I think it is probably leaking more now than using it. I'm debating fixing it vs. Trading it. What estimated cost am I looking at to fix it. Seems hard to trade it since it is probably more valuable to me to fix and drive than what I would get out of it to trade it. Your information and advice would be appreciated.