just checked pricing..WOW! NOS-3-500Z$399.95 plenty proud of their tubes at twice the price http://www.tubedepot.com/nos-3-500z.html
300's are rare and expensive. They aren't being made anymore, by anybody, so that has an impact on prices. I remember a dozen years ago when a WE 300B could fetch a couple hundred bucks. If you look at Tube Depot's prices for common tubes, they are reasonable.