What yield on cost measures
Yield on cost is this year's dividend per share divided by what you paid per share rather than what the stock trades at today. Buy at $40, collect $1.80 a year, and your yield on cost is 4.5% even if the market now prices the stock to yield 3%. The market's yield describes a purchase you could make today; yield on cost describes the one you already made.
The number only becomes interesting with time. Each dividend raise lifts it while your cost stays frozen, which is how long-held dividend growers end up paying double-digit percentages on their original purchase price.