The most closely watched lawsuit in the music industry asks this question: how much should a song on iTunes or another digital music service be worth to the performer?

The artist at the center of the suit is Eminem, but some of the biggest beneficiaries of the case may be thousands of older artists who have not released an album in decades.

Four years ago, the producers who discovered Eminem sued his record label, the Universal Music Group, over the way royalties are computed for digital music, which boils down to whether an individual song sold online should be considered a license or a sale. The difference is far from academic because, as with most artists, Eminem’s contract stipulates that he gets 50 percent of the royalties for a license but only 12 percent for a sale.

“As of now it’s worth $17 million or $20 million, but on a future accounting basis, five or 10 years from now, it could easily be a $40 million to $50 million issue,” said Joel Martin, the manager of F.B.T. Productions in Detroit, which first signed Eminem and continues to collect royalties on his music. (Marshall Mathers himself, who performs as Eminem, was not a party to the suit, although he stands to earn millions from it.)