As you did it as a neighbourly thing and don't plan to make a living out of doing it, I guess you don't necessarily want to charge commercial rates, just enough to not be taken for granted / a mug?
For odd jobs, our contractor charges £18/hour plus fuel (he fills up with our diesel on leaving.)
Or, if she is agreeable and it would be useful to you, you could take a 'cut' of the crop, say 30 bales (1/4.)
But I doubt if she'd have a problem with £1/bale if you want to charge her that much.
Personally, this year, I'd take the hay if I could, otherwise maybe ask for a round £100.