This is the 562nd most frequent Japanese word.
倍
"倍" means "times" or "multiplier," indicating an amount multiplied.
倍 is being used to mean 'twice as much,' comparing the price of the book to another.
この本は以前読んだものより値段が倍だ。
This book is twice as expensive as the one I read before.
倍 indicates multiplication or doubling of speed in this situation.
速さを倍にすることによって効率が上がる。
By doubling the speed, efficiency increases.
倍 is representing a comparison in statistical measures, expressing that the success rate is two times greater.