It ranges for me drastically based on what shops I take and how busy I am with real life. I had one six month stretch where I averaged $3500 a month, and then I sort of burnt out and moseyed along at about $400 for almost two years.
Right now, I'm at about $1000 a month and I'm okay with that.
I don't pay for gas so I never factor that expense in, thankfully.
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Plan the work. Work the plan.