Remember the 80s? then remember SODASTREAM lol...
I dont think this needs much introduction but for those that dont know, this little machine will carbonate water to which you can add flavoured syrup to make litres of fizzy drinks.
This appears to have been resurficed and you can pick up a white Genesis machine for £29.99 or the aluminium PURE version for 49.99.
This is the cheapest I have seen. Drinks and gas canisters only seem available at http://www.sodastream.co.uk.