I figure it's probably Prince because I once heard he's sold in the region of 100m. But i've also heard different figures much less than that.

Stevie Wonder I've no idea but I'd guess at least 60m, coz he was so big in the 70's and from such a young age. And he has so many albums/compilations.

Likewise Bob Dylan, who has many many albums. I'd say at least 60m but probably more because he's been going the longest.
And with his 2m or so hardcore fanbase he probably sold a couple more million recently with all those re-issues.

But if anyone here knows the figures or even cares to speculate I'd love to hear them.

Just guessing, I would say Stevie.Everybody loves him,whereas Prince & Dylan are more sort of niche-like.