In a lucky 15 you basically are making 15 bets. a €1 lucky 15 will cost you €15.
You pick 4 horses, and your bets are as follows.
4 Singles @ €1
6 Doubles @ €1
4 Trebles @ €1
1 Accumulator @ €1
If they all come up, even at short prices you would be quids in. If you only get one winner, the odds are normally between twice and 3 times the actual odds for that horse depending on the bookie. PP pays 3 times.
E.g. I have a €1 lucky 15. My first three horses lose. My last fella wins at 10/1. Paddy Power will actually pay me 30/1 on that bet, which would be a €31 return.