Sounds like a fun lecture.
Maybe one way to get your audience thinking about scarcity and resource
allocation is via a time-traveling Jean Luc Picard of Star Trek fame
trying to explain the economics of his time. You could use the segment
from 1:45 to 2:10 from
http://www.youtube.com/watch?v=TOcKGREiY30
This includes the lines:
"Money doesn't exist in the 24th century."
and
"The acquisition of wealth is no longer the driving force in our lives."
Googling "Star Trek economics" might lead you to other snippets or ideas
that you might be able to use.
Hope that helps,
Humberto Barreto