You can no longer buy a low end G4 and add a second processor. OTOH, you can buy a high end dual G4 and remove everything.
The cheapest possible dual G4 is as follows:
• Dual 1.25GHz w/ 2MB L3 Cache per processor
• 256MB DDR333 SDRAM (PC2700) -1 DIMM
• 80GB Ultra ATA drive
• Optical 1 - Combo Drive (DVD/CD-RW)
• Optical 2 - None
• ATI Radeon 9000 Pro dual-display w/64MB DDR
• Apple Pro Keyboard - U.S. English
• Mac OS - U.S. English
Subtotal $1,570.00
I think that is the same as before, but I'm not 100% sure.
I think it used to be $1270, plus $300 for dual.