First off:
Funnily enough, the cost of some oneway flights, depending on airlines, destinations and routes, will sometimes be the equivalent to, or even more, than the cost of a return flight.

I believe this may partly be due to the fact that airlines want you to fly with them, and a return ticket will see you flying more than once with them.

Yet it doesn't always work like this. I sometimes book separate domestic legs within Australia for example.

However, where are you located and are you looking up domestic or international flights? That might help you get a few more answers