Hey there!

I'm not sure, if re-scheduling fee is the right name for it, but this is what I want to ask:
I want to buy a return ticket, departing in August 2012 and returning in May 2013, but I can't do that, since the latest date I can choose at any website for a return flight is late-March.
How much would it cost, if I bought a return ticket, with the return flight scheduled for February or March and then, in a few months, contact the airline and reschedule it for May? Do you think it would be cheaper to just wait with the purchase until May or June, when May 2013 dates become available, or would the rescheduling come out cheaper?

Thanks a lot in advance!