Probably 1gp just as a common vial.
There is no mention on the capsule as a distinct item, so it is up to the DM.
This said, we can speculate on two hypothesis:
The empty capsule is part of the materials needed to craft the alchemical concotion. So it can't cost more than 5gp, a third of the price on the cheapest capsule (Table 4–3: Alchemical Capsules on page 121). Our ceiling is less than 5gp.
A vial for liquids costs 1gp. It comes with a stopper. It can be used for a magical potion so it is a very good vial for 1gp. An empty alchemical capsule shouldn't cost less than a vial. Our floor is 1gp
So the price of the capsule is something between 1gp and 5gp, heavily skewered towards 1gp.
I'd not bother with the price of empty capsules, just assuming they are part of the materials required to craft the alchemical item.