Interpret the results
Simple payback = purchase cost ÷ additional hourly income. For an upgrade, use the upgrade price and the increase over your current income.
The fastest payback and highest income can be different purchases. This compares one purchase at a time, not a sequence of reinvestments.
Assumptions
Income stays constant. Fees, collection caps, ownership changes and interruptions are excluded. Payback is measured in earning hours, not guaranteed elapsed time. There is no verified property price database.