Originally Posted by
cdn1
Germany is not as cheap specially with Euro as their currency.
Food at restaurants is quite expensive anyway
Sorry, but I find Germany to be probably the best value for food on almost anywhere in western Europe. You get decent portion size for good prices. Of course airports in Germany are the exception to the rule and there are always some places that are not so good.
Back to the original question. To stay cheap you have several options.
T2 airside non-schengen you have Burger king
T2 landside at the top of the terminal you have McDonalds (and others around)
T1 landside in the "Airport city mall" (it's in the basement, best to go looking for the rental car centre and it is right there) you have many different take away/quick and cheap(er) options. Including;
McDonalds
KFC
Thong Thai
Nordsee (fish restaurant)
and on the same level is a supermarket (mentioned above)
Scattered around you have places like "Kamps" and "Quickers" which are bakeries and do snacks, Sandwiches, etc.
T1 A airside: This is pretty poor and I find most places are not so good and expensive for what they are. But you can get a Sandwich or similar.
T1 B airside non-schengen: you have another McDonalds.
But, given your layover I would do as suggested and go into town. The S-Bahn train will take 20mins each way (allow min 45 in case of delays) and costs a little under 10 euros return.
Go to Hauptwache which you can do sometimes without changing (sometimes you have to change at the central station/ Hauptbahnhof). This the centre of town so lots of food options, including Subway at "Große Eschenheimer Str. 45" which is 2-3 min walk away. And you can then do some siteseeing etc. to waste some time.