Jno quotes �1
plus tax as the cheapest fares available. Ryanair actually have fairly frequent offers of �1
including all taxes and charges (except a �5 per flight card handling fee) to a wide range of destinations. For example, three days ago I took a day trip to Germany. My flights were charged at �1 each way, plus the card handling fees, making a total of �12 for my return fares from Stansted to Hahn airport.
Ryanair have a fairly standard list of where their really cheap flights from Stansted go to. They're all of the ones currently shown as �5.00 (inc taxes & charges, but excluding the card handling fee) on this list:
http://www.ryanair.com/site/EN/cheapflights.ph p?dest=BHD&base=STN
The list from Luton is more limited. (Again, look for the flights currently priced at �5):
http://www.ryanair.com/site/EN/cheapflights.ph p?dest=BZR&base=LTN
. . .and there are only a few �5 bargain flights out of Gatwick:
http://www.ryanair.com/site/EN/cheapflights.ph p?dest=DUB&base=LGW
You need to be flexible with dates and times to find exactly where those cheap fares really are. With some destinations (e.g. Dublin) the majority of flights will be really cheap. With other destinations (e.g. Frankfurt-Hahn) it's usually quite easy to find the cheap flights. But destinations such as Marseille-MP2 will probably only have a few cheap flights available.
To keep track of Ryanair's best bargains, register here:
http://www.ryanair.com/si