Oh, I had no idea. I thought the amount was only rewarded once per character. Thanks!
Actually, with matches set to one round each, I think Beginner sets the reward to 3K and Easy sets it to 4K, but Normal and above only reward 5K. Someone correct me if I'm wrong.
In any case, using the suggestion to take advantage of Stylish Mode, I tested a method to grind World Dollars automatically for a short time today and I'm fairly pleased with the results. I went to Episode Mode, changed the Rounds to 1 and the Difficulty to Beginner, and chose Ky. From there, I changed my button settings to have X use Slash, and used my fight stick's turbo mode to mash X while the button is held down.
I had to set it up on my lunch break from work, so I didn't get the chance to supervise it much. But when I came back from work today a few hours later, I had 30K more World Dollars than I had when I left, so the system must work somewhat.
I'd like to test some other characters to see if they yield more consistent results, because I'm fairly certain Ky didn't complete Episode Mode every attempt. I'll make a post about it here once I know more if anyone's curious.