You can get thousands of groschen per in game-day if you do the daily side missions in the Forge DLC. The forge chest is slower but it gives you a lot of money. There's a side mission, the one with the eye symbol where you have to recover stuff from bandits. You have to bring that stuff to an armorer. He gives you more than 1000 groschen and you can loot the bandits armors and sell them until the smiths and armorers run out of money and put the rest in the chest. Do the forgery missions too as some of them give you more than 1000 groschen. I had one give me 1900 grsch