In general, economists define the "opportunity
cost" of any good or service as the value
of all the other goods or services that we must give up
in order to produce it.
The idea is that, in order to increase the production of
gadgets, we must use up resources that could otherwise be
used to produce food. We give up the opportunity to produce
a relatively large amount of food. The opportunity cost
of any decision consists of everything we must give up in
order to carry out that decision (as, the opportunity cost
of the decision to increase the output of gadgets in the
model economy consists of the food the model economy must
give up as a result).