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