Snow Leopard is sold as an upgrade only. The $29 is for users currently using Leopard that want to upgrade to Snow Leopard.
That being said, the secret is that the $29 Snow Leopard upgrade disk can be used just fine to clean install Snow Leopard on any system, and I think Archive and Install Tiger systems.
Tiger is being sold for so much because people will pay it, and Apple has this problem with not always dropping the price of old stock. So, Tiger, which cost $129 new, is still going for that amount, because... the world is crazy.
Leopard is the highest level OS that PPC Macs can use, so it (at least to me) makes a little sense that its price stays a little high, but it also should have dropped quite a bit by now.
If you have an intel Mac, spend the $30 on Snow Leopard (or $60 on the 5 pack) and be happy.