But the performance the Athlon core produces could also make the Thunderbird more expensive to manufacture than the Coppermine. [snip] Adding 256 Kbytes of cache to the Athlon die would boost its cost by about 25%, according to Keith Diefendorff, an analyst with MicroDesign Resources.

AMD has pretty consistently undercut Intel in the price/mhz war, things might get a little hairy with the Thunderbird and Duron chips though.