A component basket has been created at Reichelt; total costs of all components (excluding PCB and RS-bus feedback) is below 12 Euro. Lower prices are certainly possible; other Internet shops I've used in the past and which I can recommend include Conrad, Elpro and Darisus. Note that these shops are all located in Germany and charge different shipping costs. For outside Germany, companies like mouser.com should certainly be considered, since they sell too private customers and offer a huge collection.