1: 40 - 100 bales per acre depending on when it was last grazed, what species of grass are there and of course, the weather.
2: £12 / acre to mow, £6 / acre to turn & rake, 50p / bale to bale. Expect to pay more if you only have a small area to do.
3: For quality, not quantity aim for June, for quantity not quality aim for July.
Just remember that hay making is probably the most weather crucial job on the farm and you could end up with a load of bullock hay and a bill for £1.50 / bale.