For 錢應該花在刀口上, 錢 and 刀口 are so different things... why associate 錢 with 刀口? For 錢應該花在刀口上。刀口…… and spending money. How is spending money related to 刀口?
Look here: http://www.zdic.net/c/0/3f/99108.htm

刀口 means the right spot 花 means spend

钱要花在刀口上 You should spend/use your money where it is most needed.