I'm going to replace the sight glass on my sand filter because it's become really discolored. I've been searching on-line, but I'm not sure why some of these units are so $$$! Will i regret buying a cheaper one? I've found some on amazon ( http://www.amazon.com/Hayward--Line-Sig ... 69&sr=1-16 ) for around $30, but I've seen some close to $100. What's the difference? Why would someone pay so much for a different type?