I am not up on prices but I do agree with buying in 100 litre bags.
Just a thought from my experience; Vermiculite normally comes in three grades Large medium and fine.
Large is basically for compost improvement, medium for cuttings and fine for seed sowing.
I find medium a good buy simply because it is more versatile. You can put a bit extra in if you want to improve compost, its fine for cuttings and if you seive it (its quite dusty) you get a nice sowing medium.
Just thought I would mention it!! ::)