Can't speak for your balers over there, but in Western Canada our bales are generally 50-75 lb (22-35 kg). The heavier bales are often spoiled - baled with too high a moisture level, so one tends to be a bit wary of them. They're easy to pick out if you're loading a bunch of bales, you get a good feel for the weight.
I always say 50 small bales to the tonne. size can vary though,depending on how much screw you put on the baler dependant on grass species and dryness.
i reckon 50 bales is a good measure so if its £100 a tonne then its £2 a bale A big hesston bale is about 18-20 small bales a big round bale about 15-18 small bales.
According to the farmers forums they normally look to make small bales about 25 kg BUT lots of lady horse owners moan they are too heavy even when told they are better value than lighter ones.
So lighter ones (say 15kg) may seem like a swizz but as long as they are priced accordingly (somewhat cheaper but not directly proportionately cos of the price of fuel for tractor, baling twine, effort of loading/unloading and storage etc, so will be a bit more than 3/5 of the cost).
Personally (we make our own hay for the sheep, made it for the first time last year), the first bales we made weighed about 5kg and the last ones about 25kg (we had got the hang of the baler twine tightening by then....:-O