cost of traveling around south america?

hi! i've had this serious idea of traveling around south america for about two years now. i probably wouldn't leave the united states for another two years or so, but i'm sure it'll take me that long to save up all the money i need. anyway, i would like to travel around south america with my fiance (eventually husband), but i have no idea how much money it will cost.

so here are some of my questions:
what is the best city to fly into?
is there a certain time of the year where it would be cheaper to fly and less hectic with tourists?
how much would it cost to backpack and live rather cheap for 3 to 6 months?
is it easy to find a short term job? (i know we'd need visas for each of us)
would you recommend getting a bike as a form of transportation? would it be easy for each of us to find a bike to buy?

any other advice you're willing to offer would be much appreciated.

thanks!