选择one，意味着one 是同位语。后面跟了两个定语：1.she can use  2.at a reasonable price。

选择which，意味着是定语从句。如果按照网友添加成分（is）之后，选择which 也是正确的。

I'm looking for a present for my mother's birthday, one she can use and at a reasonable price. （同位语）

I'm looking for a present for my mother's birthday, which she can use and is at a reasonable price. （定语从句）

这两句除了在结构上有所不同外，意思基本相同。