Be good
I flew MANY times on one-way tickets and on return flights of round-trip tickets. I was never pulled out of the security line, simply asked a few questions by the border guard officer at the window. Things like – why are you here (I was learning or my husband was learning), how long are you staying. It was a little nerve wracking, but not a big deal. I think that unless it was really extreme circumstances (like they suspect you of smuggling) they will issue you the standard, no-cost 3 month tourist visa and you will be expected to renew it in the Interior Ministry offices if you want to stay longer than that.
No, I did not have to pay because it was fully refundable. Just make sure you have a credit card on you that has open credit for such a charge since those are usually pretty pricey tickets.