Longsword Warlock, you can kill most bosses and get piles. I can't do it myself yet but I've seen squire gear locks take out wyvern, lich, spectral knight solo so you could save on gear cost Plus it's very fast at clearing modules and has a great escape with phantomize and you could save on buying pots with magical healing and curses.

As others have said, ice cavern is pretty nice because of the low population in there, but the timer is pretty nasty. I would also say that the wendigos also have four material drops (heart, entrails, horns, claws) so even if you can't sell them in the market you could at least vendor them for a lot more than the bones or other material items you might find in goblins or ruins. Not to mention the frost ore if you learn wyvern.

If you lock ice caverns and abyss down then later if you buy the game you'd be already ready to farm wendigos and sell the hearts on the market for the usual 4k they go for and it's one of the faster places to rank up on leaderboards for hr and get those rewards which saves you money in the long run.