The best way possible is by getting advanced wingsticks. (For those who haven't got this far, it isn't much of a spoiler) Once you put in disc two, follow the mission line until you have acsess to walk around and not be interrupted by people. Go upstairs and across the way of Mayor Redstones office there is a store owned by a lady called Jani. Look through her items until you find the 'Advanced Wingsticks'. Buy this plan and create some wingsticks. (Current windsticks will automatically become advanced) They are created the same as normal ones. (Buy the parts, trust me) Buying the parts will save you $24 dollars everytime you make some. Once you have enough, go back to Wellspring by taking the safe passage and and changing discs when prompt.
Now, for the cash making. Head to Mutant Bash and start your Re-Run. The entire time you are playing, use only your wingsticks. You will need at least 40 everytime you enter. Advanced lets you hit up to 3 enemies at a time and has a better chance of it surviving a bounce off walls and the ground. Once you repeat this enough you'll start getting 100% Accuracy which means more cash for you. You're looking at spending $102 on wingsticks each run. But also making at least $750 profit in the end, even if you make the 'Mutant Jackpot' in the bonus round. The best part? You'll in average do a run in 2:30:00 almost everytime. Do this as many times as you like.