I have rented cars to transit between NY / FL. Sometimes the rate is reasonable ($20 per day), other times they want $250. As I understand it, the rate is lower if you are moving the car to a place where cars are needed or relieving the source of too many cars, and conversely the rate is higher if they have enough at the destination.
Is there a seasonal supply / demand rule of thumb (generalization) that predicts when is a good time to rent a car to go from FL to NY and vice versa?
It will depend on the specific car company and city in Florida but late spring seems to be when many good one-way deals out of Florida pop up as the tourist season dies down and the cars presumably brought in for all the tourists flying down are redistributed to other areas with higher summer rental car demand.
Here are some examples of one-way rentals out of Florida from previous years:
I know for a fact that Enterprise Florida locations do offer a specific deal around March-April for anyone who will return the car outside of Florida. I would go to the specific branch and see what's available as I was told this a couple years ago by someone from Enterprise Corporate and I didn't really have use for it at the time.
Some signs would indicate January and February may be bad times, and summer very good:
The number of elderly temporary residents included in the household surveys fluctuated considerably over the course of the year, peaking at 10%–12% of elderly survey respondents in January and February and declining to less than 1% during the summer (Table 1). This seasonal pattern was consistent with prior expectations and with findings reported elsewhere (e.g., Hogan & Steinnes, 1996; Krout, 1983; McHugh & Mings, 1991; Truly, 2002). By using these proportions and a 2005 estimate of almost 5.1 million permanent residents aged 55 or older, we estimated that there were approximately 698,000 snowbirds in Florida at the peak of the 2005 snowbird season but only 30,000 during the late summer.