To summarize
#365, Mom owes $37,000 at 5.875%, which they already refinanced from about 9% with a bankruptcy on their record. If I put $9,000 a year into Mom's mortgage, we will pay it off in 24 months, and save $4,500 in interest, so in a way that is a 25% return in 2 years.
Otherwise, Mom will pay off her house in 5 years and 11 months.
The interest rate is less important when you aggressively pay down a debt, but I still transferred my credit card balance when I was paying it off. I don't know if I ever calculated how much I was saving, but after pulling numbers from several different sites: If Mom refinances for 15 years at 3.199%, but continues to make the same payments, she will pay off her mortgage in 5 years and 5 months, saving $2,774.48. If she refinances, continues paying the same amount, and I pay an extra $750 a month, we will save something like $5,450 total.