Let's find some examples. A Google search on "dell linux" took me to
a Dell page featuring Ubuntu 8.04. Two selections: if uncertain, see
Windows systems, else see Ubuntu systems.

Linux system, a Dell Inspiron 530n, dual-core 2.2 GHz, with an LCD
monitor, with Ubuntu 8.04, starting price US$478 - $50 = $428.

Same system but labeled Dell Inspiron 530s, dual-core 2.2 GHz, with an
LCD monitor, with Vista, starting price US$499 - $50 = $449. Add $150
for Windows XP.

Some other price check comparisons turned up similar minor price
differentials.

A $21 discount for Ubuntu instead of Vista isn't very compelling
since one can buy the system with Vista then install Ubuntu as a
second OS. The $150 premium for WinXP was a surprise to me.

The 4 systems (HP/Compaq) I bought new this year were all 64-bit,
three were dual-core, Vista was pre-installed, and the prices ranged
from US$199 to US$299. Three now triple-boot, one double-boots.