Taking down monolith bosses can earn you a good amount early on, but the best strategy is to sell items to vendors in the towns.
You can also unlock memetic specializations that allow you to craft high-value items. For instance, I focus on crafting silver and gold ingots. Silver ingots go for 6,000 each, and gold ingots sell for 12,000. Most vendors have a 50,000 currency limit per week, with one vendor having a 25,000 limit, if I recall correctly. With five vendors available, that adds up to about 225,000 you can make each week. Right now, I’m sitting at a bit over 500k.