For a 3/4 day break you are likely to be doing one leg near the weekend. For the cheap fares you would ideally like to be travelling on Wednesday, or sometime mid week. Same is true of long haul flights. Midweek can save you a fortune.
The buget airlines bank on getting a cheap leg and an expensive leg for most customers. They get to stick the cheap leg in the Ad's but the average of the two is more accurate. If you are very flexible you can travel pretty cheaply.
I had a wierd situation once trying to change a return date on a flight from the states. It was cheaper to buy a return ticket to Ireland than buy a one way to Ireland, or try to change the return date on the existing ticket.
Return cheaper than one way. Airlines are a mystery to me.
-Rd