I feel like I know the game enough to write this. To be able to do this you need to be at least level 5 (but then you need lot of potions) and have one of the lower tier spells or abilities (lower tier means taking less than 10 mp or 10 sta) spells/abilities a bit leveled up (lvl 4+).

Now, I'm doing this using Fire Stone spell, but of course you may use any other (don't expect magic arrow to work well though unless you'll make it extremely high level). No need to waste higher-tier spells and abilities for this as there are higher tier mobs around that may attack you and you should use these then.

The whole goldfarm is simple - kill zombies. They spawn near castle, both on the road to riverbell/moun camp/rockside and in the ruins (on the other side of the castle). For me, they go in two hits of Fire Stone which takes only 4mp so it's nice. I've also leveled it up a bit since it's my main spell as it takes low mana, goes well with character I roleplay and is quite powerful.

I've noticed zombies drop between 0 to 10 gold (may be wrong on the upper limit) so you'll rack up pretty fast.

At the time of writing, going into the ruins is especially lucrative if you care only about gold and not exp since there are 3 guards there so all you need to do is wait around it a bit and cleanup (collect gold dropped by zombies). That may not be the case forever though so if you read this and it doesn't work that way anymore, you've been warned.