The cheapest place for the Megaman 11w GU10 is www.ryness.co.uk: See direct link below (it's not easy to find those Megamans on the web site!). You can get a 10 pack for £75, including VAT and free post+packaging... an excellent deal - you'll pay at least a tenner a bulb on all other web sites.

http://www.ryness.co.uk/ProductDetails.aspx?categoryID=823&productID=5268&category4ID=998

The Megaman 11w GU10s are an able replacement for 50W Halogens. But I would recommend trying out one or two first before spending your life savings on a bulk purchase. The light is slightly whiter than a halogen, and not quite as strong - you do not get the "spotlight" effect the same, with more of a flood fill. Having said that, we quite like the effect, and are quickly getting used to it (the halogens seem very yellow in comparison!).