I want to buy a giftfor my mother,---at a good price but not

答案：one.
翻译：我想要为我妈妈买件礼物,一件价格尚可但又不贵的礼物.
1.如果使用which,则逗号后为非限制性定语从句.
但是该从句没有谓语动词is,是不能成为一个句子的.
即答案可以是：which is at a good price but not expensive
= a gift is at a good price but not expensive.
2.选用one = a gift,它做前句中a gift的同位语,而后面的at a good price but not expensive为后置定语,修饰one.
即：I want to buy a gift for my mother,one at a good price but not expensive.
= I want to buy a gift for my mother,a gift at a good price but not expensive.