What's interesting is that you qualify the question with a beginning year of 1607, right before the founding of Jamestown. Part of the reason Virginia expanded westward as it did is due to not the land being cheap, but the land being not available -- the first settlers made claim to large tracts around the James River, forcing the newcomers who would stake a claim to move further upriver. The increasing European population brought conflict with the Native Americans, who did not understand the concept of Real Estate. As native populations decreased, Europeans moved further inland. Eventually, Virginia split politically East and West, or Tidewater and Piedmont. Those further inland had no common interests with those by the coast; when the Confederacy was formed, Virginia remained intact until 1861, when the six counties in the western part of the state seceded from Virginia, joined the Union by applying for statehood, and became known as West Virginia.
That is a massive question indeed. Another thing to consider is that fact that when Britain began expanding across the Appalachian Mountain range, it ran into trouble with the Iroquois Confederacy, and more native resistance in general in hostile terrain. The French exploited this by arming the native tribes, and Britain attempted to restrict settlement in order to appease them after the French and Indian War. This caused tension with the poorer Scots-Irish settlers forced to seek land in the rugged hill country.
Later in US history, also consider the connection between westward expansion, the rapid growth of slavery after 1800, and the growth in tension between southern and northern interests that ultimately led to the Civil War. The war may well have been delayed if there had been a slower move west, or no expansion in that direction at all.
This is a VERY broad question that can not be fully answered here. Some ideas for you:
- It allowed the US to have a larger population and attracted immigrants
- It took pressure off the economy in the East -- the frontier gave a "safety valve" where poorer people could go make a life for themselves.
- Related to this is a famous theory by a man called Frederick Jackson Turner. He said that the presence of "the frontier" shaped American culture by making them more independent, self-reliant and inventive.
- It, of course, affected relations with Native Americans during both periods. The expansion forced whites and Indians to come into conflict and led to the near-extermination of the Indians.
- It led to the idea of Manifest Destiny and the war with Mexico.
These are just a few of the ways that this process affected the colonies and later the US.