Human expansion has almost always been in the form of planting seed colonies that get most of their growth in situ.

Taking large numbers of people off of earth is certainly cost prohibitive now. With a space elevator that made it vastly more affordable than at present, relatively large numbers could visit space. By relatively large numbers I mean orders of magnitude more than now, but still paltry when compared with the population of earth.

Still, once the seeds are planted off earth and the new frontier's limitless resources become accessible in a bootstrap process, off earth population will eventually dwarf the earth's. (But the vast majority will be born off earth).

I think one of the big questions in conjunction with life expansion off planet is what form of life will it be. We have entered the era of self evolution, and artificial life creation. Will we be able to call the life forms that colonize off earth homo sapiens, or will they be some new replacement for our species--"homo spaciens".