When you've exhausted all of the other upgrades in the game, why not spend that hard earned cash on dressing up like a chicken (30,000 coins), a Dread Knight (30,000 coins), a marathon-running knight (20,000 coins), or a Mayor Knight (20,000 coins).
Upgrade to the first two loot abilities first, and you'll make enough money to quickly upgrade the other areas.
The potion that gives you extra loot for killing enemies is a good purchase if you quickly want to get some more funds.
There's a secret room filled with loot, but it's always hidden behind crumbling walls. Grab a shield and run into these walls to find the bonus area.
If you've picked up a shield, you definitely want to charge headfirst into any obstacles for a tasty multiplier. You won't die, you'll smash them to a thousand pieces. Likewise, swords will allow you to slice through enemies and grab a reward for doing so.
Unless a trap is stopping you from doing so, head into each corner in the middle lane. That will give you the best chance of avoiding any spikes or barriers that might be spawning on the left or right hand sides of the dungeons.