I am glad this is useful to you.
Similar to pachinko balls, they take a certain percentage as commission when converting the coins to tokens. Based on my calculation, it is about 12%. I also noticed different cities has different percentage. And it’s the same for pachinko balls, definitely not 2.5 yen as you mentioned, that’s too low.

Yeah the parlour takes 2 of your coins even before playing.

Apparently there are indications, according to the guidebook, but it is still pure randomness and luck.