Sorry, I'm being presumptuous here: are you referring to charter flights and/or discount airlines? If you travel with one of the �majors,� i.e., BA, Malev, etc., the meal service is complimentary. However, there are carriers that do charge for meal service, such as Iberia. I'm not sure of your destination. But I would suggest that some of the payable meals are seldom true value-for-money. If your flight is of a short distance, you might prefer to save your money to apply at your ultimate destination. And instead, on your flight, take a small packet of crackers or crisps.
For an interesting example of meal types on offer, have a browse at:
http://www.airlinemeals.net/meals/CzechAirline s.html
Bon Appetite and Bon Voyage!