Thank you for bringing up this contentious topic. Some people believe that it is immoral to charge for a kitchen.
They are wrong - it is a service and an expensive one at that! Furthermore, my experience with letting guests use my kitchen only lowers my ratings due to increased scrutiny.
Dependent on your pricing ( budget vs luxury) I think $10/day is very fair.
That way, if they leave kitchen a mess, take over the refridgerator or leave a lot of garbage you can hire a housekeeper to minimize the damage.
If you have that wonderful cook (clean, considerate) , you could also rebate.
I am assuming you have minimal cooking facilities for free - a kettle, microwave.
Once again thank you for broaching this subject. The myriad of choices is what makes airbnb strong and give guests what they want and the ability to pay for what they want.