I suppose you could make it small but then what do you plan on printing with it. Having a smaller printer means you have a smaller print area. I always seem to run out of room with my current printers so going smaller would be questionable to me. If you plan on printing really small objects I could see this but why limit yourself. I would be more than happy to help you out if you would like to build one.
To build a printer for 100GBP($153) is going to be hard to do. The cheapest i have been able to build a printer for was around $300 and thats because i was able to print all the parts off another printer i owned. The printer Billy posted is a little over your budget bud not far off.
