The cheapest solution is to buy a Firewire external 5.25" enclosure and pop a standard Apple Superdrive (like the Pioneer 115D) into it. I did this recently to give my old iMac G4 a DVD writer. Firewire is better for the PowerPC Macs - not sure if your Mac mini is a G4 - or you could use a USB2 enclosure if the Mac mini is an Intel processor version. I got my enclosure from Micro Direct for £17 and the Pioneer 115D from eBay at £22 - total price: £39. The benefit of getting a proper Apple Superdrive is that it works with Apple's DVD Player application and iDVD better than certain high street brand external DVD drives. Before deciding to go down the Superdrive route, I tried a Firewire LaCie DVD burner we have at work, but Apple's DVD player wouldn't recognise it, nor would iDVD either (although you can burn your iDVD projects to a disk image, then use Roxio Toast to burn that separately). But the external Superdrive is recognised fine in iDVD.