In principle this isnt difficult but you would need to plan out how you would want to display it for given amounts. If it was me the way I would do it would be to first create a string of the value
var money_string = str(money)
and after this you can base some logic on the length of your string for instance
if money_string.length() > 4:
print(money_string.left(1) + "." + money_string.right(1) + "k")
would be one simplistic way to do this but may not be the way you want to show it. Try reading through the documentation here
https://docs.godotengine.org/en/stable/classes/class_string.html?highlight=string
and you can then plan out your logic.