Cheap in the sense that we have lots of them that can be put to work for a pittance. I suspect there would be no shortage of fully developed humans willing to go on "one way" missions to Mars or further. I know I would do it myself.

(dredmorbius: I am in fact referring to humans. Using microbial life is an interesting idea in its own right however.)

I suspect GP was referring to microbial or other simpler forms of life.