Wow, that money score on guitar is gut wrenching. Lol, at what point do you just stick your Sim in a corner and hash out a motherlode every few hours? You'd probably be making less, lol!
Although I do gotta say, best money/time ratio is without a doubt photography.
Data: Base sim, lvl 10 photography. Trait: Photographer's eye. Skill challenges completed: photolog, shutternut, architectural eye.
Simply put, the benefit of photography is you're not logging simoleon scores by the hour, but by the minute. It is the least time constrained money maker possible.
Scenario: Vacationing Shang Simla. first thing in the morning, from the vantage point of my balcony: boom, Dragon Cove (4,300); boom, Academy (2,200); boom, chinese garden (2,200); boom, my jade figurines set (1,800 - I keep them set out on a coffee table on the balcony). 10,500 simoleons in the scope of about ten minutes.
Now the kick-back, of course, is that this will be my most lucrative ten minutes of the day. My 10,500 in ten minutes does not equate to 63,000 in an hour. And while the day WILL offer me a few more good cash-ins, from just buzzing around home and town (Chinese Market, Vase set, etc.), y revenue stream for the day as pretty much plateaued at this point.
But the back drop to all this, is if you don't try to force yourself beyond the plateau, point blank your Sim is only working 10 minutes a day.