So I'm starting in a campaign at 6th level, and I used some of my starting money to buy a +1 adamantine nodachi for around 4,000gp (we had a party member with craft magic arms and armor) I want to put Keen on it, that would put the base price in enchantments up to 8,000, (4,000 with craft magic arms and armor) would the price from +1 carry over to the new base price? And would adding Keen cost 3,000gp, 4,000gp, 7,000gp, or 8,000?
You would subtract the cost of the +1 weapon from the cost of a +2 weapon, and pay the remainder, in this case, a total of 6,000 GP.
A +1 Adamantine nodachi would cost an additional +5,000 GP over the weapon cost, and to add Keen to it would then cost an additional 6,000 GP.
A +1 weapon by itself would add 2,000 GP to the base cost, and a +2 weapon would cost an additional 8,000 over the base price of the weapon.
8000 - 2000 = 6000 GP.
The adamantine special material does not impact the magic weapon pricing for enhancements.