Not only are fexible tickets more expensive, they can easily be more than twice low-cost tickets or twice two one-way tickets.

Your best bet may be to buy a regular low-cost round trip ticket for a return date that may work. In the event you cannot return on the purchased date, you can check wether the airline will allow you to change the return flight for a fee or lose the return portion and purchase a new ticket for the return. In that event, you might gamble and purchase a second round trip with the intent of just using the first half or purchase a one-way.

As noted above, airlines don't like you to purchase a round trip ticket with the intent of throwing the second portion out but a 1 or 2 time event will likely go unnoticed.