We think it's really important to buy the very best straw for calves, because they'll pick on at eating it.
So we buy barley straw from untreated crops / fields, usually has quite a bit of grass and if we're lucky some flower/seed heads in it.
It ain't cheap, though. £25 / round bale, £4.25 per small square bale, delivered.
East side of Scotland you may get it
much cheaper - I've heard about £5 per big round bale, and seen it left rotting in fields.
I bed my four down in their large pen every other night; I guess I use about three small square bales a week. I think for a pen of 10 you'd be using a small square bale every night, at least. Figure on 8-10 small square bales per big round bale, so a little less than a big round bale per week - maybe 4 big round bales per 5 weeks, something like that.
You'll need to think about muck disposal...