And then fourteen pounds a month for three years.

1.个人认为,a month在此结构中是后置定语,修饰限定fourteen pounds.
2.再如：
1) My salary is 4,000 yuan a month.我薪资是每月4000元.
4,000 yuan在句中作表语,但是a month做其后置定语,表明/限定是“每个月的4000元”.
2）I return to my hometown twice a month.
twice在句子做状语,但是a month做其后置定语,表明/限定是“每个月的两次”.