...the breeder doesn't have to do anything for you. If the snake came up sick a few days after you got her, then I think the breeder should replace her at no cost to you. But, if the snake was healthy when you got her and you've had her for a month now, then there's really no one at blame. Sometimes these things just happen, unfortunately. In this case, the offer of a snake at half price is an attempt to meet in the middle. The breeder will take a loss for you.
This happened to me once with a Tangerine Honduran. About a week after I got her, she ate but then refused every meal after that. Also, after about 2 weeks with me, she started leaving little piles of "goo" in her cage and "acting" funny. Her movements were a little jerky, like a male mating dance. I kept calling the guy I bought her from saying that something was wrong and I wasn't sure what. He kept insisting that she was ok, and not to worry. The last time I called him he said, "Do you want to bring her buy so I can look at her?" I said, "YES!" I brought her by and again he said she was fine. But, while I was there and holding her she started becoming more and more relaxed. I turned her over in my hand and she stayed that way! He looked at me, looked at her, looked at me again and said, "Ok...now THAT is odd." I left her there and she died that night.
For a replacement, he offered me another snake at 50%. After that, I didn't want another Honduran so I took an adult male snow corn. He was priced at $100 so I got him for $50. It was either take his offer or I was out an animal and my money.
But, the whole thing still irks me...I spent $85 on the Tangerine and another $50 on the snow, so the snow ended up a 135 dollar animal...which is too much for a male snow.
My point is, do what you feel comfortable with. If you want to approach the seller about replacing your girl for free, do it but do it carefully...otherwise you may lose your 50% offer too.