eBay is the cheapest there's a dealer on there has the plx, aems and innovate widebands which are some of the most popular and reliable systems and they're the cheapest I could find with 24h delivery free. They were based in London, I had the plx dm6 as it was slightly more expensive but boasted massive features over the others including capacitive touch screen and plots graphs in real time and can be linked with up to 6 different modules of your choice to display anything from boost, rpm to oil pressures etc, and also supported data loging