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Depending on where he/she lives it's not. I've been there. Full time jobs are not paying enough. It all depends on your personal situation. Some developers I worked with used to live with parents and they thought their salary was great, I couldn't do that so I found I couldn't save anything.

Possibly, but even in San Francisco the median wage is $78K.

120K is not a lot at all if op lives in a coastal city (NY/SF/Boston/LA). Especially if OP is the sole provider of the family.

You could also consider asking for a raise. Your boss might be more comfortable giving you a raise than having you distracted by and spending energy on projects outside of work.