the cost of freedom

i've seen a few bumper stickers or whatnot declaring that "freedom is not free." i don't really understand this. at the korean war veteran's memorial in washington d.c. there is an engraving that says this, too.

i'm not sure if this is the truth (as something that would exist outside our (human created) society) or if it's a social creation. if it is the latter, is it created to make us feel more ok about war, about dying, using literally billions of dollars to promote this militant idea of freedom? if the former is true, what is the cost of freedom? lives lost? dollars spent? time? effort? is freedom part of what is natural and, perhaps to extrapolate that: innate? well, if it's not free, who sells it? is this something i can buy or barter for?

i personally believe that freedom (or lack thereof) is a creation, a state of mind or state of being. i believe that society can tell you that you are not free, and even physically restrain you (i.e. in jail), but then we get into different definitions of freedom. here's a poem, by ho chi minh:

Although they have tightly bound my arms and legs,
All over the mountain I hear the song of birds,
And the forest is filled
with the perfume of spring-flowers.
Who can prevent me from freely enoying these,
which take from the long journery
a little of its lonliness?

this is the freedom that i relate to. i do not yet understand the freedom which is not free.