According to the d20srd, 20 arrows cost 1 gp.
The thread here indicates DC 12 as a consensus:
I would cost out arrows individually at a base price of 2 cp each to make. The actual number is something like 1.6, but 2 is just so much easier for calculations.
I'd use a DC 12 for the skill check. This is because arrows are "complex items" according to the Craft description and making them correctly is a skilled job - yes, as skilled as making the bow itself.
Base Price of Arrow (1): 5 cp
Cost To Make: 2 cp
Craft DC: 12
Craft Check (assuming +10 and a roll of 10) [It is possible to take 10 on a craft check, and silly not to]: 20 * 12 = 240 (cp)
I'd assume "a day" represents 12 hours of hard graft, which means that the whole thing can be broken down nicely into hours. Wanna spend 1 hour making arrows? Fine, assuming the above numbers, you make 4 arrows and it costs 8 cp (buying 4 would cost you 2 sp, so it's not quite a third).
You still need to make the normal arrow as per above.
Base Price of MW Arrow (1): 600 cp (price taken from the equipment chapter)
Cost To Make: 200 cp
Craft DC: 20
Craft Check (assuming +10 and a role of 10): 20 * 20 = 400 (cp)
MW Arrows/day: 2 thirds (or 1.3 days to complete 1)
A bow takes DC 12 to make, a composite bow takes DC 15. Because arrows aren't different for composite or regular bows and are "complex" items, 12 is the consensus there. DC 15 if you want to count them equivalent to "martial thrown weapons" but it seems counterintuitive to have an arrow be more difficult to make than the bow.
Thread on community.wizards.com saying the same thing.