I get the feeling that 40 taper is the most readily available and cheapest for medium sized machines. 30 taper costs more due to less industrial usage. HSK is for big and really high speed machines and big pockets - unless you are running >10krpm in a large, powerful spindle there's no advantage for the likes of us. R8 is a US activity and more readily available over there but it's only really suitable for small to medium machines - created by Bridgeport for their machines.

I can get a wide range of good quality ISO40 tooling for my Shizuoka for around £20-25 a pop. My Bridgeport has ISO30 and the holders are more difficult to find and cost a fair bit more. I don't get close to challenging either of those with the work I do.