No you are wrong rether, it is not more than $100 more expensive.
X800 xl: $278.99
6600 gt: $179.00
The x800 xl is only $99.99 more expensive than the 6600 gt. And he has enough money to afford it. It is a much better deal and bang for your buck than the 6600 gt. Sure the 6600 gt is mainstream and will run games well, but the x800 xl is high-end and MUCH more powerful than the 6600 gt. The x800 xl is obviously the better buy.
For only $99.99 more than the 6600 GT you are getting DOUBLE the bus width (256bit to 128 bit), DOUBLE the VRAM (256MB GDDR3 to 128MB GDDR3), DOUBLE the vertex pipelines (6 to 3), and DOUBLE the pixel pipelines (16 to 8)
If you have enough money to afford the extra $99.99 I don't see how you can argue that the 6600 gt is the better value, when clearly it is not...