Percy is talking about how we as people want to buy the perception of the thing rather than going and seeing it for what it truly is. The Grand Canyon is an example of a thing that people go to simply because of the name recognition and popularity. Percy is trying to show us that we are more interested in saying "We went to the Grand Canyon" than we are actually seeing the beautiful place it is.

Not everyone values the same exact things. Each human being sees, feels, and reacts differently. I think Percy is trying to explain that we as humans value different things based on the relativity that it has to us. For some they value the Grand Canyon because that's what they like and some value insulin because that's what they need.