1999 Plymouth Voyager Driver's Seat Belt Won't Extend

Not sure how common this problem is - I've never had it on any other vehicle. The driver's seat belt will not extend easily most of the time. Pulling on the best seems to engage the "catch" that's designed to hold during a crash. Repeated gentle pulls will eventually coax out about 6-8 inches at a time - almost like reeling in a fish in reverse. The belt has to be held out until the next section will extend, and so on, until you get enough to buckle it. If you let go and the belt retracts, you have to start over. Odd thing: it's much worse when the weather is cold. Obviously I have to replace it, but my question is, how difficult and/or expensive would this repair be?

Thanks in advance!