I paid only 5 pounds for the book ,_______I expected it woul

选择 not so much as
解析：
1.从句子成分来说,有两点需要注意：
1）not so much as I expected it would cost 是 5 pounds 的同位语,much 是不定代词,指代 the price,因此也该改为 not so much the price as,或者改为其他形式的同位语：
* that is to say it is not so much as I expected it would cost.
* namely it is not so much as I expected it would cost.
* i.e.it is not so much as I expected it would cost.
* 同位语从句：that it is not so much as I expected it would cost.
2）不定代词 much 是 as I expected it would cost 从句中谓语动词 cost 的逻辑宾语.
2.从词义来说,5 pounds 指的是价格.含有数词和复数单位的名词词组用来表示价格、重量、距离等度量,其实是不可数概念.如果用作主语,要视作单数,需要用单数动词；如果要指代,要用表示不可数概念的人称代词 it 或不定代词 much.
3.其他选项分析：
A.not as many as：many 指代可数名词.
C.cheaperthan：是形容词短语,用来做陪衬性状语,但是 cheaper 与后面的句子 as I expected it would cost 没有必然的关联.
D.not so expensive：也是形容词短语,但是一是与后面没有必然的关联,二是缺连词 so,不完整.