Earned $20,000 from mission completion rewards.
At the end of each mission, you are awarded money to spend at the shop. The amount depends on how far into the campaign you are and what medal you were awarded for it. For instance the last mission 'Farnsworth' rewards $2,000 for being awarded the Bronze medal.
By completing all missions up until this point, you should unlock this on the mission 'Volcano' in the last act 'Pacific'.