Usually in prehm-Duke Fishron farming queen bee is the best option of money farming (without building a large elaborate money farm)
Her treasure bags usually drop 1-2 pieces of vanity gear, beenades, beeswax, honeycomb (sometimes) and on higher difficulties hive pack, relic and pet sell for gold as well. Boss spawns for QB aren't hard to get either as the recipe is very cheap, just sell the drops and insta plat
once you kill Duke Fishron, he's the best thing to grind money for, he usually drops 50-80 gold, his mount and excess loot sell well too (and his master mode and expert mode drops sell well too). A high damage setup combined with Flairon or Bubblegun usually shreds duke. Truffle worms are also easy to farm with a water candle, or making a farm (simple just mine tunnels in mushroom biome)
To make the most of your money, make a high happiness housing setup for goblin, I usually do a house over shimmer with goblin, mechanic and dye trader (just goblin and dye trader before so), for low reforge costs and pylon setup for shimmer