So, in simple math terms with rounding... your extra 8hrs/wk or 32hrs/month got you an additional ~$60K. So, in your situation/location/etc... not taking into account your PDs limits/etc, one could conceivably pull down an additional ~$120K by doing 16 hours more per week. If your training program is a predominantly 60hrs/wk program, you end up working 76hrs/wk.... still well below what other subspecialty trainees and attendings work.
As others (BD, MedicineDoc, CabinBuilder) have noted, each area/program has different opportunities. The key ingredients are
1. what is your baseline residency workload/structure,
2. level of work one is willing to do over and above residency,
3. What restrictions are there limiting your moonlighting.
Long and short, while some medical students may believe it is impossible to moonlight for an additional 150-180K, it is actually possible and is done.