The price of oil at present is higher than that of oil in the past
这句是没错的.
如果用more expensive 前面就不用price.
即The oil is more expensive than
因为expensive本身就指价钱.
例：
The car is much more expensive than that one.
这辆车比那辆车贵的多.
高中文凭senior high diploma
更多的用High School Diploma,很少看到你的用法,但我只能确定我写的更常用.