Many car rental companies do this, and it is very much a supply and demand situation. So it varies every year, and cannot be booked in advance. Generally it isn't for cross-country, though; PHX cars are going to LAX, and FL cars are going to NYC+200 miles.

Once I was paid USD 50 to rent a Lincoln for 5 days and move it from LAX to SFO; but that was an unusually sweet deal. The downside is that the mileage allowance is generally tight (plenty for point-to-point but not for touring) and additional miles are charged at a punitive rate ($1/mile is typical) as are any additional days.

In the past Hertz and Budget were good sources for this on the west coast (may not apply to FL though, also it often varies by individual location, so there might be a difference between FLL and MIA for example).