You will need to buy everything that is available in the store from the Main Menu: Characters, Costumes, Upgrades, Attributes, etc... You are looking at roughly $100,000 to $150,000 to purchase everything from the store.
If you need a few bucks or need to get money fairly quickly. You can replay level 5 "No Cheese, Please," as you get roughly $10,000. You get $120 for each Secret Service Agent headshot.
NOTE: If you replay any level you must collect all secondary objectives and complete the level. Do not just quit out.