The formula I came up with works great for determining the baseline:
it's roughly (max spell level affectable)^2*300
so for lesser, it's 3^2*300 (2,700), rounded up a smidge to 2,750
regular is 6^2*300 (10,800), rounded down a smidge to 10,750
greater is 9^2*300 (24,300), rounded down a smidge to 24,250
Then you multiply that by (metamagic level)^2-(metamagic level-1)^2
so for maximize, you multiply by 5 (3^2-2^2), giving you
lesser maximize: 13,750
maximize: 53,750
greater maximize: 121,250
then you add 250 to the last number (probably just to make the number rounder, maybe the intrinsic value of the rod itself)
so 14K, 54K, and 121.5K are the prices for a +3 metamagic rod (right out of the book).
The above formulas work for all 12 examples of rods in the SRD *except* the two I mentioned. I'm sure someone did it for some balance reason, but why not make the regular and greater quickening ones also more expensive? It just isn't consistent and just another lame WotCism (that's my new term for their "weird" stuff).