The cheapest version is the one you can buy from shapeways.com; search for B5m.
http://www.shapeways.com/search?q=b5m
About £55 delivered for 4 parts needed from shapeways, you also need a 3 gear motor plate, and the more recent top shaft/bearings I believe.
A mate at our club is running it, no problems with accuracy in terms of gearbox internals but he did snap the brace that goes up to rear tower...though it was a massive full speed end of straight into wall shunt to be fair.
I've enquired about two others via Facebook and while they're both beautifully made, they are quite expensive. But when you've personally put all that work into R&D I think you're entitled to charge a higher price, and people are buying them.
I'm still hoping AE do one that also enables shocks to front of rear arms...otherwise I might have to consider a YZ2 or an XB2...!