Since I am in family medicine residency, please allow me to expound.....as far as the extra income during training is concerned, of course this varies. For example, one of the senior residents at our program pulled in close to 130k last year. This guy is in his 40's, and has three teenage kids, so, has to work hard to support his family. He regularly moonlights at a nearby rural ER at 100 bucks an hour. Also, during weekdays, he works at the after hours urgent care clinic at 75 bucks an hour.
This is of course , a bit extreme....however, most of the other senior residents are pulling in close to 80-90k without too much stress. One must remember that most FP programs have relatively relaxed call schedules during 2nd and 3rd year. Our 2nd and 3rd years take inhouse call approximately 3 times a month.
Like the OP mentioned, there is no limit to the financial renumeration in this field, depending upon one's drive and interest.
Many a times, employers will give you a generous loan repayment package after residency, especially in smaller/midsize towns. Many of the recent graduates got upto 100k in repayment, averaged over 4 years.
hth