If you hypothetically had a successful side project, that can probably be parlayed into "I could do something like this for you, where it will be substantially

If you hypothetically had a successful side project, that can probably be parlayed into "I could do something like this for you, where it will be substantially more effective because it will take advantage of the X, Y, and Z advantages your business has that mine didn't." This. If you have a side project that's bringing in, say, $5000/year, and you do a few things to it such that it's now bringing in $5250/year, you've boosted revenue by 5%. Now, go find a prospective client with a business bringing in $5MM/year, and explain to them how what you will do for them has the potential to increase revenue by $250k/year. Sure, it might not scale 1:1, but boosting it by even $100k/year should justify paying you, oh, $50k for the engagement.