I like the Mustang idea/adventure and encourage that idea, but...
I just ran a one-way Hertz rental from these locations with AAA discount and the weekly prices are:
Pickup Time Mon, Feb 16, 2015 at 12:00 PM
Return Time Mon, Feb 23, 2015 at 09:00 AM
Pickup Location Courtyard Springfield HLE
Return Location Los Angeles - Beverly Hills Body and Paint HLE
Ford Focus or similar
1 week at 229.50 USD
229.50 USD
Vehicle Licensing Cost Recovery
2.17 USD
Energy Surcharge
1.49 USD
Included
Unlimited Free Miles Included
Vehicle Subtotal
233.16 USD
Taxes
23.32 USD
Total
256.48 USD
One of the secrets is that "HLE" stands for Hertz Local Edition.
These are the "suburban", non-airport locations. By not using the airport locations you will avoid a lot of additional taxes, fees and jacked rates.
Here is the same rental from Reagan National to LAX
Pickup Time Mon, Feb 16, 2015 at 12:00 PM
Return Time Mon, Feb 23, 2015 at 09:00 AM
Pickup Location Ronald Reagan National Airport
Return Location Los Angeles International Airport
Ford Focus or similar
Rate Details
1 week at 1274.99 USD
1274.99 USD
Airport concession fee recovery
142.06 USD
VEH LIC COST RECVRY
2.17 USD
Customer Contract Fee
17.50 USD
Energy Surcharge
1.49 USD
Included
Unlimited Free Miles Included
Vehicle Subtotal
1438.21 USD
Taxes
143.83 USD
Total
1582.04 USD
3,000 miles traveled in a Crown Vic getting 20mpg will require 150 gallons @ $2.30 each will be $345 in gas.
Move that to 40 mpg (doubled) in a Focus and the gas price will be half or $172.5
Said another way...
Spend $256.48 and you will save $172.5.
Renting a car will cost you $83.93
You can alter the mpg and gas prices but the moral of the exercise is that about $100 of expense will get you a rental for 7 days.