Cost of the tire isn't directly relative to their longevity. Some of the most expensive tires are ones that don't last nearly as long -- they're made of softer, "grippier" materials for better performance in curves, especially. COnversely, a lot of the less-expensive tires are harder, wearing better, but not having the same performance characteristics.
The trick is trying to find that sweet spot between performance and wear. Not very easy! I've been very happy with the handling on my P250 with the stock Kenda on the rear (on its second), which is cheaper, and wears well, and the stickier Pirelli GTS23 on the front (my second front tire, first Pirelli, which has better control and feel than the stock Kenda had).
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