Firstly Apple has discontinued the plastic MacBook's, the only ranges still running are the MacBook Pro's and MacBook Air's. Now to the main question. The MacBook Pro's are made from an aluminium unibody which means the cost of manufacture and materials is higher. This is not only due to the aluminium but also the glass screen and so on. Also in general (with the occasional exception), the MacBook Pro's have better hardware such as processors, RAM and HDD's. The MacBook Pro's are much better built and in general faster and in my own opinion worth the extra.