My machine is the MakerBot Thing-O-Matic which costs about $1250 and is in kit form, everything you need to get up an running. If you are looking to build your own then you probably want to be looking at the RepRap project. Building your own does save a lot of money but you have to get all the parts yourself and spend some time learning what you need and how to build it.
I would recommend building a RepRap Prusa Mendel, they are the easiest and cheapest version to build. You can get a full set of plastic parts off eBay for around £75 (search for 'prusa mendel' or 'reprap') and most of the other parts are from a hardware store. Depending on where you get the parts, you're looking at under £300 to build the whole thing.