From the House Republican Energy and Commerc Committee:

Under the Republican proposal to allow Americans to buy health insurance across state lines, a family in Massachusetts, which has already adopted many of the health reform policies in Democrats’ bills, could see its health insurance premiums reduced by over 61 percent if they were allowed to purchase family coverage from North Carolina.
 
“Every day, millions of Americans purchase millions of items and services from other states – either by driving across the border to a favorite store, by picking up the telephone to place an order, or by placing an order from one of the thousands of retailers on the Internet. Why can’t we do that for health care?” said U.S. Rep. Joe Barton, R-Texas. “Republicans support giving you the right to purchase a quality health insurance of your choice. The federal government denies you this ability even if the innovative health insurance plan that best fits your needs happens to be headquartered in a different state.”
 
American families could see real change by allowing purchasing insurance across state lines. For example:
 
A resident of California could see his or her health insurance premiums reduced by about 50 percent if he or she could purchase health insurance in neighboring Oregon.
 
A resident of New York could see his or her health insurance premiums reduced by 57 percent if he or she could purchase health insurance in neighboring Pennsylvania.