A ticket out of the continent is enough.
Post 1:
I have just returned from traveling in South America. I went through 6 countries and the only time I ever had to show proof of having an onward ticket was when I bought a one way flight from Tulcan (Ecu) to Cali (col). Not once did an immigration official ever ask for this.
Post 2:
I have crossed countless borders in South America and never once have I been asked to show proof of onward travel. I think this is a law on the books so they can block someone questionable, but, when a tourist shows up with all their documents in order and carrying cash or credit cards, they are welcomed with open arms.
Post 3:
I went in and out of countries all over (probably 20+ border crossings) the continent and was never asked for proof of an onward ticket. A ticket out of the continent should be good enough if you were ever asked.
Post 4:
Countries have these regulations to keep out the riffraff. They are used to deny entrance to poor people who are clearly coming in to work. They are not used against tourists. Tourists are welcomed with open arms.If you have concerns, plan out your itinerary and print it out. Then, when the airlines asks about a return ticket at check-in (something, I repeat, I was never asked about), show them your itinerary and explain you will be leaving by bus on such-and-such date. That, with a credit card to show that you have the means to leave the country will satisfy any immigration agent who happens to ask. Or call the airlines and explain what is going on and ask them to put a note on your record so you will not be denied boarding.