The Witcher 3's last expansion, Blood & Wine, is not kind on Geralt's finances, at least not initially. The cost of moving up to Grandmaster gear is steep, as are the prices on upgrades to your vineyard home.
UPDATE: Seems this will only work for a save game created before the most recent patch. If you try and start a new game now, this won't work, sorry!
You could do things the hard way, Witching around the countryside, or you could save yourself some grind and try this money exploit (while it still works).
Here's the gist:
First, you need to return to White Orchard, go see Willis the Dwarf smith and buy everything off him so that he has a few thousands coins in his purse. Then head to the two buildings highlighted in the pic above. Once there, use your Witcher Senses and you'll see a beehive. Cast Aard and knock it off. Then you're going to keep using Aard to push the beehive all the way back into White Orchard.
It's easier to do this at night, since there won't be many people around. Anyway, keep pushing until you reach Willis (he'll be outside his shop no matter the time). Once you get near to his house, it gets a little tricky, as you need to push the beehive close enough to him to deal damage, while staying far enough away from him so that you can actually cast Aard (it won't activate if you're too close)
Once you get it close enough, you'll see the bees start to do damage to Willis. After a few hits, he'll drop dead. Do not mourn him. Just sit down, meditate for an hour and...
He's back! For a few seconds, anyway, because with the beehive still there, this Willis is not long for this world either. And when he dies, meditate, watch, then repeat as many times as you want.
When you're done, there'll be a mountain of Willis corpses out the front of his store, and this is where the exploit comes in: every single one of these bodies will be carrying everything Willis was selling in his store when he died, along with the cash he had on hand. So if Willis had 3000 coins and you sit through ten deaths, then you'll be able to pick up 30,000.
He should also have some rare crafting elements and jewels on him too, so make sure to loot those as well.
And that's it! At time of posting, this exploit worked on v1.21, but as with all Witcher exploits, be warned: you do this at your own risk. CD Projekt Red have ways of knowing who has done this...