Depends on what Boss Star they have.
Boss Stars are unlocked starting in the Mountain levels. The higher the Boss Star level, the greater starting gold and income they have. Currently there are four available Boss Stars, with a fifth one unreleased. (You can check the Thang editor to see this). Also, higher Boss Star level grants the hero to summon better units. (There’s a guide somewhere on the forum.)
A rough conversion scale is as follows:
startingGold = 20 * bossStarLevel
income (gold/sec) = 2^(bossStarLevel - 2)