Good question! Premium fluorocarbon tippet is about $15 per 30 yards (50 cents per yard) and premium fluorocarbon spinning line is about $20 per 200 yards (10 cents per yard).
I've heard people say that fly fishing tippet is stronger per diameter and a more consistent diameter. It's also hard to find spinning line in small diameters. But is fly fishing tippet enough better to justify 5 times the cost?
I've heard of more fisherman, and especially nymph fisherman who use more tippet, who are trying spinning line. So I picked up a 200 yard spool of premium fluorocarbon line recently. It's about 4.5X and I plan to give it a good workout nymph fishing in our winter / early season.