I think the best thing is to call around and get prices but if I had to recommend a dealer, I'd say Messicks
New Holland Parts
They don't show the part in stock or a price and the only thing I found online for that part number was WADE TRACTOR & EQUIPMENT INC Griffin GA dated 1/02/07 and (Quoting from their book) "SBA322050640 PINION, DRIVE $510.96" don't know if this is dealer cost or retail...at that time. This is also assuming you are needing the main pinion in the differential, Messicks parts page lists a "bevel pinion" on the end of each axle for $173.78 each.