while $85 is only a fraction of the $700, its still $85... Also when you say that the only cost difference is in the CPU prices, what about the memory costs? For a 2500+ you would want DDR400 since thats about as much as you can hope for, but on a P4 system would some DDR433 or DDR466 be in order? That adds the cost too... but then again these new P4s outdo anything from AMD...
Athlon XP 2500+ @ 2225MHz (12.5x178 @1.850v), Volcano 7 w/Smart Fan2, A7N8X Standard Rev. 1.06, Kingston HyperX PC3000 512MB @177MHz, Radeon 9700Pro, SB Live 5.1