Very, very easy to get the books up. And so are most of the other retailers (Barnes and Noble, Kobo, Google, etc.) Apple make it a bit harder (by trying to enforce you have to have one of their devices to upload it) but still easily doable.
The hard bit is getting anyone to buy it!
I make a few hundred pounds on fallow months, mostly from my novels, but this can rise to £1500-2000 if I'm working on the promotion. Compared to other authors I know, that's peanuts.
I hang on to my marginal claim to kudos of having been the 5th best-selling science-fiction author in the world (according to Amazon). It was a for a few hours!