Complex sentence: Thasian power extended to the mainland, where the Thasians owned gold mines even more valuable than those of the island; their combined annual revenues amounted to between 200 and 300 talents.
Split into simpler sentences: Thasian power extended to the mainland, where the Thasians owned gold mines even more valuable than those on the island. The combined annual revenues of all these mines amounted to between 200 and 300 talents.