Niagara even combines shipping on Amazon so when you buy more stuff the shipping doesn't go up hardly at all. I usually combine about 5-6 things when I make an order to them (seems like every week or so as I'm always restoring bikes) and shipping is rarely over $10-12. That makes the set of tires well under $20. I don't care about durability that much as I'm always working on bikes the changing of the tires is nothing. If they last only half as long I'm still 2x ahead on price.
The only real niggle I have with the Kenda/Chen Shins is all the hairy nubbins they come with. They drive me crazy. Other tires have them too but it seems these have 2-3x more of them and longer too. I usually spend 4-5 minutes with a razor blade cutting them all off on each tires. I'm a bit OCD -but I hate hearing them whiszzzzhing against the fender stays.