Buzz
A lot depends on the room and the other side games they offer apart from Bingo. A bulk of the revenue from normal bingo rooms comes from slots and videopoker. I guess your players have just started playing and you need to build up the database. It is a game of volumes.
To answer your question, yes 15 Euors per player @25% is a good start. Most bingo players play about 20-30 Euros per month. Butthey play for a long time. Considering that you are doing well.