Let me tell you a story... was at the feed store one day and saw a duck limping, so I caught her a saw that she had a piece of twine wrapped around her leg. the foot was dead, and required amputation, but I was in no position to deal with that, so I took her to the local avian vet and asked them if they wanted her, they probably didn't, but they did take her and said they would treat her. This is what I think you need to do with your duck because this lameness has been going on for so long now that he needs proper medical treatment. Since you cannot afford a $100 visit, I'm assuming you can't afford proper drugs either, yes? Not trying to make you feel bad, but you really need to do what's best for your duck.
-Kathy