Here it is, then. it's 'finished'. Keyboard is a USB Cherry G81-7000, with the card reader gubbins taken out, a whole load of dremeling, the original USB repurposed as the power lead (with the keyboard connected via a short hacked-up old micro USB cable with the micro end taken off, an internal USB soundcard (even though I never use it), and, um, that's it. Still having problems with power - plug a mouse into the handy built-in hub at the back, and you get sticky keys. That'll either have to be fixed later on, or I guess I'm going to have to bite the bullet and get a powered hub.
Cost (not including the glue gun I bought):
Keyboard - 7 euros.
Raspi - 32 euros
Soundcard - 3? euros
Total - 42 euros. Bargain. It needs some cosmetic work done and finishing off, so the *final* cost will probably go up a bit, but it's usable right now.