Westward expansion had a huge impact on economic life in the USA during this time.

First of all, westward expansion gave the US a huge new "breadbasket."  The Great Plains were by far the best agricultural country (once technology like good plows had been developed that could deal with its soil) for grain in the US.  This allowed fewer farmers to feed more people, thus freeing more Americans for other kinds of work.

Second, westward expansion opened up new mining territories.  This included California's gold rush, of course, but also the silver mines of Idaho and Nevada and other mineral deposits.  This dramatically increased the US store of natural resources.  It also led to the push for bimetallism in US currency.

Finally, the westward expansion helped to create industrialization.  The opening of the West led to the need for more railroads, which helped to push the industrialization of the North (because of the need for rails, rolling stock, etc).  This was, of course, one of the most important changes in US economic history.