[R] Combining math and variables in expression

[socrates]

I am trying to get the following line in a plot margin using mtext:

100% Area = 120.000 km^2

Where I intend that "100% Area =" is text, 120.000 is a number that varies
according to different data, and km^2 should be a neat
km-with-superscript-2. 
The expression function fails me, since it apparently cannot coerce an
expression when a variable is involved. 
In the R help file there is an example using either bquote or substitute to
accomplish like things, but these are sufficiently different from my
objective and I am becoming more and more confused by every further example
I stumble on. 
This should be easy. Can anyone help me prevent spending anoter 2.5 hours on
the matter? Thanks.
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