When something is in high demand it becomes scarce and when things are scarce, the price usually rises.
The amount of employment will go down, as with less diversity of crop growth, there will be less need of specialized skills and machinery will be used more often as it will be more economical to the individual growers to use it.
Pests (such as insects) which feed on these crops will increase in number, resulting in less of other types of insect species, and so reduce biodiversity all the way through the food chain.
Crop diseases will become more common, as they will have more freedom of movement.
Soil quality will become poorer as there will be less crop rotation, so certain minerals will be striped from it.
Chemical fertilizer usage will increase to replace certain soil nutrients.
It will become harder to purchase locally grown foods as the land will be used for one crop, so will increase road transportation and pollution.
More genetically modified crop growth will be undertaken in an attempt to control some of the other problems.
Soil erosion will increase due to intensive farming...