I think the reason they cost so much would be the fact that they're just not standard products, plus, iirc, you have to change the helix angle on left hand threads by a tiny amount (so the thread leans the other way).
If you've got access to a lathe though, they shouldn't be too hard to make a load of. Get some M8 countersunk bolts, take the threads off them and then proceed as below:

ThreadingMethods_zps0aab93a4-1.jpg