Hello! In this guide, I will show you how to obtain the filthy rich achievement. Obviously, there are some spoilers ahead. The guide also assumes that you know how to get all three endings, and that you are stuck at earning the last few (around 10) coins to get the achievement. I won't give you an exact route to follow, but with the information below, I'm sure you'll be able to figure out what you are missing :)

First, let's take a look at the different ways to earn money in the game:

💰= selling at pawn shop, 🔪= murdering in cold blood

So, in total there are 360 coins. This means that you have 10 coins to spend as you wish. If you still have less than 340 coins, you should take another look at the map. However, even if you have been searching the map thoroughly, you might still come up short of the 350 coins necessary to be filthy rich. Most likely, this is due to some of the following things:

• Rats are worth 0.2 coins. However, the game will always round money down. For example, selling 5 rats will get you 1 coin, but selling 6, 7, 8 or 9 rats will also get you 1 coin. Thus, you should only sell at the pawn shop when the number of rats you have is a multiple of 5.
  
• With the rodent cape buff, you will lose rats when you have 20 in your inventory.
  
• You can sell two old keys at the pawn shop for 3 coins each. One can be obtained by talking to the old man, the other by killing the critter holding it next to your spawn. Using a key to open the gate will consume it, so if you want to maximize the the number of coins you have, you cannot use the gate during the run.
  Additionally, the old man will only give you his key if you don't have one already, so you need to either
  
  • sell the critter's key first and then obtain the old man's key, or
    
  • talk to the old man before killing the critter, which allows you to carry both keys at once.
• Selling carrots at the market will give you 2 coins per carrot instead of the 1 coin the fisherman gives you (thanks to Holmi for the comment!)

TL;DR: kill everything, don't carry 20 rats and sell both old keys. Happy looping!