On me
Is used when we want to pay for someone's food and drinks.
"Lunch is on me today".

What if somebody tells me that the lunch is "on you" today.
What should my answer be?
Can I say "with pleasure"? Or just "OK". Are there any other possible answers??

And what if I want to answer in a funny way? What should I say?

Thanks!

Question

On me
Is used when we want to pay for someone's food and drinks.
"Lunch is on me today".

What if somebody tells me that the lunch is "on you" today. 
What should my answer be? 
Can I say "with pleasure"? Or just "OK". Are there any other possible answers??

And what if I want to answer in a funny way? What should I say?

Thanks!

Answer

It's pretty rare to make someone treat you to a meal, unless maybe they owe you a favour/lost a bet/it's your birthday or something. You can respond in any way you want just like when someone says "you're paying for the meal today!" There's no particular phrase you need to use to respond.