For all practical purposes, Mr H, I would call that 0.5 cubic metres. Thickness of 100mm, plus an allowance for extra needed where the hardcore base is not even. (0.43 cu.metre plus allowance.)
If you can get a 1 tonne bag, then go for it. Should be about right. Check this out. Normally, "Dumpy bags" are around 850kg ........ 0.85 cu.m.
Terminology varies around the country. I guess by "ballast", you mean what we refer to as "all-in mix". 20mm chippings and sand mixed together in one bag, in a ratio of 2:1 ....... chippings:sand.
That's the stuff. Just add cement and water. A ratio of 6:1 would be plenty strong enough. (Mix:cement).
The critical thing with shed bases isn't the mix etc, but the "substrate". In simple terms............. what's it sitting on?
Clanad's idea with surplus ready-mix is a really good one. For some reason, American companies tend to be rather more helpful than ours. Over here, some guy in an office may not feel inclined to help out. Often, if someone orders a 5 cu.m. load from a 6 cu.m. truck, they make him pay for the missing bit anyway.
Another way is to hire a mixer truck that actually carries dry mix and cement (separate) on a truck with a mixer on board. They can batch as much as you like, with no surplus to throw away.