The DS (bigboy) started out at $149, dropped to $129 when the system showed signs of tanking. When the Lite arrived, it just took over the existing price point.
If you compare the original DS to the Lite, it was a pretty drastic visual overhaul.
The PSP is thinner ... but it still retains that LONG body. Also that thumb-buster D pad is still intact. The extended battery is nice, but in most part, the PSP2 delivers stuff nobody asked for.