Throw good money after bad
Definition 1 of 1
Pro Tip 1/3Failed Investments
Use when discussing financial decisions that continue to consume resources without returns. 
Investing in that startup was throwing good money after bad.
Pro Tip 2/3Futility Nuance
Emphasizes the pointless nature of further investment in a failing endeavor. 
Repairing this old car is just throwing good money after bad.
Pro Tip 3/3Emotional Connotation
Carries frustration or disapproval towards the continued spending or effort. 
She realized she was throwing good money after bad at the casino.