5,000,000 G may sound like quite the hefty amount, but this trophy will really pop just a little over half way through the scenario campaign if you're playing through it casually. You get more money based on clear time, enemies defeated and overall points in a stage. Enemies also drop gold in this mode, and you can equip items that increase the value of items you sell by x percent.
You can earn gold online and in arcade mode as well with each win, playing through arcade nets you a decent chunk of change but simply playing through Scenario Campaign is by far the quickest method.