The Witcher 3: Wild Hunt is a massive world with a living ecology, its own economy and, inevitably, a few wonderful ways to exploit its virtual people to make tons of easy cash quick.
Streamer WhatsMyGame posted a couple of videos detailing how, with a little tedious work, a witcher can pull in stacks of crowns. The first video, seen above, shows players where they can go to slaughter a seemingly endless supply of cows, skin them and turn them into crowns.
But that has its limits and can be time consuming.
The second video, posted below, shows how a little bit of legwork in town can turn seashells into pearls, pearls into florens and finally, florens into crowns. That exchange can net a player about 100,000 crowns in 45 minutes.
It's worth noting all of this was done on console and hasn't yet been tested on PC. That said, there are likely much easier ways to bring in cash on the PC if you're willing to mod.