I think I'd pay to go on a guided walk if it was over and above "just a walk". Like you say, if they have a specialist knowledge and the walk is sort of educational, or possibly if they had special permission to walk somewhere otherwise off-limits.
I'd also possibly pay for a guide if I wanted to do something like, eg. climb Tryfan. It's not something I'd want to do alone and I'd want someone experienced and knowledgeable with me. Having said that, if I was expected to jump Adam and Eve as well once I was up there then they'd have to pay me back, and then some
As for price... not sure. I'd be happy to pay more for the technical knowledge and guidance of a mountain guide as opposed to someone who just "knows stuff" about an area (flora, fauna, archaeology, geology, etc.) but I don't know any going rates for either, so I can't really comment on that aspect.