I would say look at the "day job" quality for your payouts for an honest job's pay. If you make earning money through a hits based system linked to a skill or group of skills, you run the risk of setting up even more debates with your players as you have seen here...
The other side of the coin is, if it's so easy for the characters to earn money with out the normal constraints (like needing a valid SIN, or license, or permits, etc) why didn't they just do that in the first place? While money shouldn't be the only reason why someone runs the shadows, as mentioned before, for some it IS the only option to make a living.
Also, unless you are looking at this as a spring board for mission ideas (as said earlier, the Mob comes by the mechanic's shop looking for protection money, etc), I wouldn't make the payouts for a day job all that rewarding, and use the time spent working against any time spent learning new skills or acquiring new gear.