The opening of frontiers always provides opportunity, as well as challenges. Many of the settlers moving into the area were looking for freedom to live their lives as they wished, which usually included the opportunity to gain land, which was the chief form of wealth in the 19th Century. Other people wanted to be able to tap into resources such as minerals in the form of iron, lead, silver and gold as well as animals such as beaver and bison. We also cannot forget the fortunes made in lumber from the old growth forest. A person who was willing to work hard could, possibly, earn a tremendous amount of wealth that those settled "back east" could not imagine.
At the same time, as these settlers moved west, they created another opportunity for merchants, doctors, teachers, and other "support workers". The miner needed somebody to produce food, tools, and all the other implements necessary to mine. Of course, the same is true for every other occupation. The true fortunes during the California Gold Rush, for example, were not made by the miners, but by the merchants selling things to the miners.
In summary, it was the unused or underused resources, either the products of the land or natural resources, that provided the economic opportunity, as it was almost like found money in that it was just "there", waiting to be developed.
The expansion of the west and south provided economic opportunities in several ways. As people began to move west and south, the railroads followed. People needed a way to get to and from the region, and the railroad companies saw this as an opportunity for growth and for profits. Additionally, as people moved south and west, businesses also followed. People needed many products as they moved south and west, and businesses, like the railroad companies, saw this as an opportunity for growth and for profit. Businesses would provide the products people needed in these regions. Railroad companies could also ship these products and make more money by doing this. Individuals also saw opportunity by moving to these regions. Some people took advantage of the offer of free land if they lived on it for five years. This was part of the Homestead Act. Many of these people became farmers. Other people moved west and south to raise cattle and sheep. Some people moved west and south to go into mining as they heard reports of large mineral deposits in these areas. Finally, some people saw opportunities for starting their own businesses. They viewed the available lands as an area for great business growth and economic development. The movement to west and south provided many economic opportunities for people who were willing to take the risk and seize these opportunities.