It looks quite good if you view it as returning somewhere between 35-50% per year on invested capital. However, it's hard to guess what shelf life of the product will be.
I would be very surprised if Geneforge 1 (which I don't think is his earliest title, but is several years old) hasn't sold more than 5 copies this year. The tail for his products is very long. Pretty flat I'd guess, but very long and since they're distributed digitally there's no reason not to keep them available, gradually dropping prices or making bundles available. Once 5 is released he could sell a "box set of the entire series" and spike sales of the old titles again.