I've been doing small consulting projects off and on for the past 20 years. It brings in between $10k to $50k/year in additional income. Average is probably closer to $20k.
How do you find work? Word of mouth, or something else?
If you don't have an existing network freelancing/consulting is going to be very difficult. I've tried the online freelance website method before while I was in college. It sucks. The clients don't want to pay even a reasonable amount and they're usually pretty shitty clients to work for.
If you have a network I'd suggest consulting. If you don't then I'd say steer clear.