I fiddled a bit with it and here are some of the numbers I noted (assuming bonus is based on surplus):

- City with Machu Picchu producing 12 surplus food, all other cities were getting +10 food from it
- City with Machu Picchu producing 16 surplus food, all other cities were getting +12 food from it

- City with Machu Picchu producing 21 surplus food, all other cities were getting +15 food from it

- City with Machu Picchu producing 40 surplus food, all other cities were getting +22 food from it

- City with Machu Picchu producing 75 surplus food, all other cities were getting +30 food from it

- City with Machu Picchu producing 185 surplus food, all other cities were getting +39 food from it

- City with Machu Picchu producing 709 surplus food, all other cities were getting +47 food from it

All the tests were made in the same turn, buyout-ing food quarters and such.

If you want a save to test it out, feel free to use this one (Machu Picchu is in Kerma).