TBH you can get the same sort of power levels out of both the begi and the fm kits, it all depends on what you add on to the basic kits.The only real ones, as mentioned, are the Begi kits, and the Flyin miata ones. The Begi is the cheaper option but the flyin miata ones seems to be running more power (obviously with more upgrades available etc). Both have to be imported which bump up the price quite a bit.
You can look on here and the bay for cheaper options to come available (like I have been doing) but these get snapped up soooo quickly even when you spend hours waiting for them to pop up. Even then these second hand kits might not be worth the price in the long run as they often need replacement bits or have bits missing etc. I have been looking now for a good few months (at the cheaper end of the market) but like I said they are rare as 9 pence. Theres a few decent kits which come up for sale but again your looking at around 2k.