I have a relative who plans to fly from NJ to CA and stay for an as yet undetermined number of months. Can he buy a round-trip ticket without providing an exact return date? If not, won't the cost of two one-way tickets be much more expensive? Would he be better off buying the round-trip and then paying to change the return date (assuming that can be done)?
Depends on the airline/timing. I've often purchased one way fares and it turns out to be the same (or less, if you book far enough in advance). JetBlue in particular prices all fares as one-ways.
If you buy a round-trip ticket, the typical airline will charge a change fee plus any difference in the airfare. Check the rules for changing flights first.
If Southwest goes to wherever you are flying, I recommend them as their fares are not different for one-way vs round-trip. Also, they do not charge change fees (just fare difference) if you make a reservation and then change it later. So, for situations like yours, they are a great choice.
Remember also that if you're booking one way trips, you're not stuck with a single carrier. You can fly whoever has the best fare at the time of booking.
Almost all airlines now have "segment" option. When I book E-W coast, there is never a difference for booking together or separately. Go with the one-way.
In the "old days", the one way was cost prohibitive. Today, it's more often just about 1/2 the cost of the ticket. This is for most airlines. The only one I have found to be poor with this has been AA-often seems higher to do one-ways.
"Unlike a regular return ticket, an open-ended airline ticket sends you to a destination on a specific date, with a flexible return date -- usually within one year of your departure date. Although the ticket may be more expensive than a set return or one-way ticket, not all airlines offer open-ended tickets, so you should ask to speak to a customer service representative about this option."
Related question: Is there an optimal time to buy that California ticket fare-wise? I know that last-minute purchases are very expensive, but how far in advance is wise? A month? Two months?
unicorn33