It also depends on if it's high, medium or low white. I still see high whites going for $500. I got my high white girl for $200 from PetSmart on a black Friday deal. They are getting cheaper though, they used to be thousands. As a side note, check out Scoria Boas. Those are hot including the price. I need to wait for them to become more common now
And I agree, now that I see better pictures, I don't think that's a burn either. Looks like a stain as well from substrate or paper or something. maybe the snake got into a cup of grape juice
Anyways since you're new, be sure to read the guide on cage set up for a 20g. It'll get you set up good.