I looked into this for exactly the same reasons a year or so back. Came to the conclusion that a few year old Mac Mini if all that is needed for app development - I'm still use to grabbing a coffee or a pint (or two) at the pub whilst waiting for compiles to finish

Unfortunately, whilst I put out the feelers for a pre owned mini, no one came up with tangible goods at the �300 ish mark, although still would be interested depending on the spec. So the new ones at �529 look quite a good option now they have had the price cut and IIRC some previous extras now included as standard. As they still not come with mouse and keyboard will standard PC style USB ones suffice to avoid paying the �80 for items that I already own? Will a Windows standard USB DVD/RW drive also work in place of the �66 superdrive?