Complex sentence: The patent yielded him a good annuity of 20 crowns and exempted him from certain taxes--he being bound in return to paint likenesses of the successive Doges of his time at the fixed price of eight crowns each.
Split into simpler sentences: The patent yielded him a good annuity of 20 crowns and exempted him from certain taxes. In return he was bound to paint likenesses of the successive Doges of his time at the fixed price of eight crowns each.