For what i've seen on ebay and the mini forum, they sell from £50-£70. Although be very weary of twin carbs, as you have to check the throttle linkage VERY regularly so they don't go out of sinc and cause the engine harm by 2 cylinders making more ore less power than the other 2, which leads to upset crank, pistons and conrods, which usually leads to a wasted engine.
If you are wanting more power, then get the 1275cc from here:
Click me
I don't need one, and look its only in carlisle :
You shouldn't need to worry about the dvla and engine conversions, as it is only altering the engine, not the car or subframe, all you have to do is tell them its bigger, but still A-series. My donor mini was a mini 1000 with a 1330cc engine in it so says 1275 so on my V5.
Hope that helped
Max