I’ve been doing some research on traveling to South America and staying for an extended period of time (working vacation). The usual sources for flight info (Kayak, Expedia, Hipmunk) don’t have any info on flights with open return dates. The other option would be to just buy a one-way ticket, then buy a return flight when I want to leave, but I’ve heard that international flights from South America are quite expensive. What would be the most cost-effective solution?
What you want is an "open return" ticket. The outgoing flight is confirmed and the return is "open". Depending on the type of ticket, the return can be up to a year after departure. You can book the return ticket after departure--subject to availability of course. Sometimes you can even change a confirmed return without penalty. You'll probably have to buy an open return ticket from a real travel agent--I've never seen them for sale through the online sites like Expedia etc.
Travel agents that specialize in a certain destination for immigrants are a good place to find these kinds of tickets. For example, if you want to go to Brazil look for a travel agent that sells tickets to Brazilians immigrants living in the USA. You can often find listings in "ethnic" newspapers that will be distributed at restaurants and shops. Student travel agencies are also a good place to check as they sell tickets to students who will be studying abroad for an indeterminate amount of time.
Open return tickets can be more expensive than simply buying a cheap fare, guessing a return date and then paying a penalty to change the date if you need to. If you choose this option, be very careful you understand the change rules to avoid paying too much.
Finally, I would be careful about only buying a one-way ticket. Most countries have an "onward travel" rule which means that you must show proof that you are planning to leave the country. Usually, this means having a return flight ticket. A bus a train ticket out of the country might be accepted, but I wouldn't count on it. This rule is not regularly enforced, but I've seen cases where travelers without a ticket out of the country have been denied boarding.
I solved this problem recently by using miles to get an award ticket. Several airlines (among them: United/Continental, and American Airlines) now allow you to book a one-way ticket for half the cost (in "miles") of a round-trip ticket. If you have the miles, this can be a great deal. I just booked a one-way ticket from California to Germany for 30,000 miles and $40.
Be aware that there is no such thing as a completely open ticket.
In my experience buying tickets from Australia there are open tickets for three months, six months, or twelve months.
There are no tickets sold with validity beyond twelve months. If you find tickets where this does not hold I would love to hear about it!
The longer the ticket is open for the more expensive it will be. You do have to put an initial date on the return portion of the ticket but it will be changeable.
Now the price to change the open/return portion of the ticket will vary considerably based on the airline and the price of the ticket. Actually it depends on the "codes" on your ticket which you probably can't read but travel agents and airlines can. Sometimes it depends on who you bought your ticket from. I changed my 6-month open ticket a couple of times last year direct with the airline and they had to check with my travel agent whether I would have to pay for the changes.
The good news is that some airlines offer free date changes for the open portion of your ticket, including stopovers. Some other airlines tell you there will be a charge but may in fact not charge for the changes. Korean air changed my dates for free last year so I'm flying with them again this year. JAL used to do it for free but last time I flew with them they told me each change would be $100 USD (this is a typical price) but when I made changes they in fact didn't charge me.
If you're going to buy your tickets online it may be difficult to ascertain for sure whether the date changes will be free or what they will cost. If the ticket site has a phone number you may be able to find out by calling them. But this is one reason why I still buy my plane tickets from travel agents so I can know all these details before they get my money.
Unless you're flying to a country you have residency all countries I know of have a policy that you must have a return or onward ticket. In practice whether this is checked depends on political things. If you are from a poor country and are visiting a rich country you are much more likely to be checked. Also if there is some "situation" between the countries. I don't think I've ever been asked for this on arrival but a friend was asked when visiting Malaysia from Australia last year. I have however been asked to show it many times when extending a visa or doing a "visa run" in Mexico. Still, I've met many travellers braver than me who only ever use one-way tickets for their long-term travel.
Very few airlines offer "Open Return" tickets like they used to, and those that do are normally far more expensive than a normal return ticket.
Normally the best option is to book a normal return ticket, but with the return date as far away as is possible (which is normally either 11 or 12 months, depending on the airline). When you are ready to use the return you will have to pay a change fee (often ~$250, but it depends on the airline) to move the return date to the date you want to fly. You may also have to pay a fare difference - especially if you're traveling at a busy time of year.
If you're looking at staying more than 12 months, you've generally got no options but to buy a one-way ticket.