I think it's an interesting question because we see a lot written about the incomes and catalogues of other stars/songwriters like the Beatles and Madonna, but never about Dylan. The Beatles catalogue has famously changed hands a couple of times, so we have a sense of how valuable it is.
Many of his songs from the 1960s are still performed, recorded, and played on the air, but whether he gets a steady stream of income from them depends on how smart he was about retaining the rights when he was a beginning musician. Dylan has always struck me as pretty damn smart, but he's also very secretive. We'll probably never know until his will is read.