Are you aware that SJ Games publishes an annual "
Report to the Stakeholders"? This is an almost unheard-of level of transparency from a gaming company. The report discusses SJ Games' revenues and prospects in enough detail for your business nerd(s) to ballpark a reasonable value.
That said, sir_pudding is exactly correct: SJ himself is the only
shareholder, and apparently has no interest in selling. He responds to comments in the thread I linked pretty regularly, so I'm sure you can find his own words on the subject.