Just math. ROI equals gain(or savings) - cost of investment all over cost of investment.
So, if the incandescent bulb you are replacing is 100 watts and the replacement is 10 watts your saving per hour of use is 90 watts. If you use that particular bulb 10 hours a day, that's a savings of 900 watthours/day. And you use it every day, that's a savings of 328,500 watthours a year (or 328.5 kilowatt hours (KWH) a year).
Now if your cost of electricity is, say, $0.11/kwh. The new bulb will save you $36.14 per year. If the life of the LED is 10 years, your total savings will be $361.40.
If the bulb cost you $15 the math is (361.40-15)/15. Result is 23.09.
I suspect, however, that what you really want to know is, "How soon is my payback?". In this case, you get full payback in a little less than 5 months. Then all of your savings from that point on are all yours for no additional cost.
Of course any differences in daily/annual use costs of electricity or the LED will impact the calculation. Also, having to replace the shorter life incandescent makes the LED an even better choice. And think of the environmental benefits of the LEDs.