You get money by blowing stuff up ($3 per object, $5-10 for vehicles) and getting the collectibles. You will get about half of this in a single playthrough, so if you're going the 1 playthrough (Hardcore first) route, you will want to keep repeating farm the rest. Also note, the value is cumulative, and you do not need $15,000 on you at a single time.
To farm money easily, load up mission 1 and proceed to the minigun section where you clear out all the enemies with the mounted gun. Use the gun to clear out all of the cars and explosives, then die. When you continue you will get to repeat the process, and you'll also still have the money from last attempt. Repeat the process as long as you want.