You're asking why the 512 SSD upgrade is $400 in the quad-core 3.5GHz iMac versus $300 in the 6-core 3.0GHz machine? It's because the quad-core machine base config is a 1TB HD and the 6-core base config is a 1TB Fusion Drive, which is a $100 upgrade from the 1TB HD. So, it's the same overall cost.