The environmental gains are nice and all - but you'll get further if you just point out the economic advantages.
- Depending how often you cook, you can save about $2,000 per year by cooking with an induction stove*
- If you replace your gas hot water heater with a solar one, you can save about $500 per year
- In a cold climate, you can save another $2,000 per year by heating your home with a reverse cycle air conditioner instead of a gas heater. And it'll keep you cool in summer.
So - several thousand dollars per year in savings. More than enough to justify the upgrade price.
(* also, a good induction stoves will run hotter than any gas stove, and you can set a lower minimum temperature as well. Best of all though at every temperature in between you'll have more control and faster temperature changes than a gas stove).