By the time there are 21 million bitcoins, I'm not sure any of the currencies that it is valued in currently will even exist, and accounting for inflation over such a long period is fraught.
Using the Big-mac index, I predict that by the time there are 21 million bitcoins, a McDonalds© Big Mac© will cost 6 satoshis.
First of all, I think by the time the last Bitcoin is mined there will no longer be something as primitive as a McDonalds Big Mac. There will also probably not be a McDonalds at that point.
However, you should be able to get a serving of food for around 6 satoshis yes.