How much could the prorata difference be for a single or partial month? Even moving from my 1999 Mazda Miata to my 2005 Crossfire my premium only went up by about $85 a YEAR!

Like the others have said, you contracted with the insurance company to cover you car for that amount of time. They did their part, you really need to do yours. Had you wrecked the Crossfire during that time they would have paid up far more than you had.