I am replacing the flywheel in my 2007 G35 4 door sedan but I can only find 12310JK20C and it says it replaces the 12310JK20B that is in my car. Is this alright? the 12310JK20B is about 200$ more which I don't understand. If anyone has some insight that be great! thanks!

Usually on a part number like that the final digit signifies which revision the item is, and yes a revision C would supersede revision B. Likely the increased cost of the older revision B item is simply due to supply and demand, they don't make it anymore so the limited quantities signify a higher cost according to the pricing algorithm used.