Most Economical Plan To Visit All 50 States

Trip 1- Northeast.
Fly into Boston, Massachusetts.  Take I-95 N  up the New Hampshire coast into Maine then cut west to the Green Mountains of Vermont.  Take I-91 down to Connecticut with a small detour to see either Providence or the Rhode Island coast.  From New Haven, CT, take the Metro North train to Grand Central Station in New York City.  Once in NYC, take the PATH train to New Jersey to see Liberty Park.  States Visited- 8

Trip 2- Mid-Atlantic-
 Fly into Washington, DC.  IMHO, even though DC is not a state, you cannot complete this challenge without a visit to our nation's capitol.  Walk across the Potomac to Arlington, Virginia.  A small detour will allow you to catch the eastern panhandle of West Virginia and Harper's Ferry.  Take I-70 to Baltimore then pick up I-95 to Philadelphia, PA with a stop in Wilmington, Delaware.  States Visited-5.  Total-13.

Trip 3-  Southern Appalachia-
Fly to Atlanta, GA then cut north on I-75 to Chattanooga, TN.  Head back east to North Carolina and see the Great Smoky Mountain NP.  Take the Blue Ridge Parkway up to Mt. Mitchell.  From Asheville, head south into upstate South Carolina before completing your loop to Atlanta.  States Visited- 4.  Total-17

Trip 4- Gulf Coast-
This one is easy.  New Orleans, LA to Pensacola, FL passing through Mississippi and Alabama.  I certainly recommend doing a lot more than just passing through but this adds another 4, which brings the total to 21.  I recommend Ship Island in MS and Mobile, AL if you stick to the coast.

Trip 5- Great Lakes-
Fly into Minneapolis, MN and cut south to Iowa to catch a state park in Decorah.  Next, go through Wisconsin.  From there, choose your option depending on the amount of time you have.  You could stop in Milwaukee then cut south to Chicago, IL.  From Chicago, there is Indiana Dunes NP just across the border as well as several state parks on the Michigan side.  However, I would recommend Door County, Wisconsin and Michigan's Upper Peninsula instead if you have time.  At any rate, that's another 6 states and 27 in total.

Trip 6- Southwest
Fly to Las Vegas, Nevada and take I-15 to St. George Utah, passing through Arizona.  If your time is limited, you'll only be in Arizona briefly but you can stop for a hike at the Virgin River Canyon to make it legit.  If you have an extra day or two, Horseshoe Bend and the North Rim of the Grand Canyon are not too far away.  3 more states and a total of 30.

Trip 7- Rocky Mountains-
Fly to Denver.  Rocky Mountain NP is a must see.  From there, it's just a short drive up to Cheyenne, Wyoming.  Then cut east to Scottsbluff Monument in Nebraska.  From Scottsbluff, it's only 3 hours north to Mt. Rushmore in South Dakota and I figure, this is the best chance to hit North Dakota as well.  5 states, which brings the total to 35.

Trip 8- Southwest 2
Start in Albuquerque, New Mexico then go east on I-40 to Amarillo, Texas.  Palo Duro Canyon is a must see there.  From there, it's a bit of a stretch but you can pass through and spend a night in the Oklahoma panhandle on your way to Dodge City or Liberal, Kansas. Both Kansas towns are 25-30K people so there will surely be nice sit down restaurants and hotels.  There is not much in the way of towns in the Oklahoma panhandle but there is a nice state park so that makes it legit. States Visited-4.  Total 39.

Trip 9- Pacific Northwest-
Break up a trip to Alaska with stop in Seattle.  From Seattle, head south to Oregon and see Mt. Hood, Cannon Beach and Multnomah Falls near Portland.  If possible, I highly recommend going into British Columbia, Canada.  States Visited- 3.  Total 42

Trip 10- Hawaii and So-Cal-
You can't drive to Hawaii and if you live on the east coast, the jet lag coming back could be severe.  Why not break up the trip back by spending a couple days in the Los Angeles area?
States Visited- 2.  Total 44.

Trip 11- Yellowstone- Fly into Jackson, Wyoming (repeat) and see Teton and Yellowstone.  Take the Beartooth Hwy into Montana then cut southwest into Idaho.  2 new states.  Total 46.

Trip 12- Mississippi River.
Repeat Tennessee with a start in Memphis.  Cross the river into Arkansas then cut north on I-55 into Missouri.  I'd recommend seeing Little Rock and St. Louis.  2 new states.  Total 48

Trip 13- Triangle.
This last one is easy.  Start and end in Cincinnati, Ohio.  Cross the river into Kentucky and stop in Louisville and Lexington before going north back to Cincy.
Indianapolis is worth a look as well if you need Indiana.

Of course, there is a lot more to see than what I've posted and several states can't be done justice simply by following these directions but I can't think of a more economical way to Visit All 50 States.