[robhack]

Not everything has to be about ROI, especially when it comes to art, in my opinion. For example, I might want to write a fiction book even though I know I won't sell any, just because it's something that inspires me and want to have done in my life.

You could also mitigate the ROI by learning both the guitar to get your quick ROI boost, while learning the cello when you want to get serious (not that you can't get serious on the guitar, but you might like the cello better and want to get serious on it instead).