Asset and value of a corp

According to the description of 'value', 'This is the value of the corporation, computed as the price of one share, times the number of shares. This is generally accepted as the market value as this is the price that has to be paid to purchase all outstanding shares of the company.'

Question: how is this calculated? If a corp owns a lot of materials, will its value increase correspondingly? Also, I don't know why local or common market contracts reduce corp value and asset.....