Hi Martin,
We were at the Rila last week. It was 42 levs for a safety deposit box. I was a bit wary of leaving passports and all our English money in them at reception, but everything was fine. You ask for them, get whatever you want from them and they lock them away and give you the key.
Had a fantastic time by the way. Enjoy!
Just got back on Saturday, we were charged 8 lev per day which is around £2.90 ish. Never had any problems just make sure you're there early in the morning to get one as they ran out while we were there, I heard a woman complaining to the people at the desk about not being able to get a box.