longarm,
I did not know you could say external, Mac, and inexpensive in the same sentence.
I do not know of any straight DVD-ROM drives, but ASUS has a slim line drive. It is a USB 2.0/Firewire CD-RW/DVD-ROM Combo Drive.
The price in the U.S. is $159.99 for the SCB2408-D. (Figure the price should be around £159.99 the way pricing in the U.K. seems to go.)
Take Care,
Richard