Couldn't the book cost 7.5k and one has 6.5 and the other has 0.5? Along those lines, isn't anything in the range of costing 7->8 (non-inclusive) acceptable (e.g. 0.9k and 6.9k)?
current 10 kopek is worth 0.0013$
They probably come from time when 0.5 kopek was the smallest coin.
#1 problem doesn't have single non-zero solution if the smallest coin has larger or smaller.