First off, a DM may rule that an arrow does not have enough metal to justify using mithral. Outside of that yes, they would cost 75gp each or 1501gp for 20 arrows.
Basically it it is not worth it to create buy (or create) mithral arrows. Your better off buying (or creating) the silver arrows. Normally you lose ammunition when it hits and 50% that miss. If you have a nice DM, he might allow you to have mithral arrows that basically never break. But as soon as you start enchanting them, you may have problems-DM may decide you have to enchant them individually.
Not sure what you mean by save DC. The Craft DC to create anything with special materials is at least DC 20 because they have to be masterwork quality.
The item does not have a break DC but mithral has a hardness of 15 and would have a hitpoints of 30 (normal arrows have hardness 5 Hit points 5). Non-magical unattended items always fail their saving throw. Attended items use the players saving throw.
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If your referring to the save DC of spells, the formula is DC 10+ level of the spell + your bonus for the relevant ability (intelligence for wizard, Charisma for Bards, paladin, sorcerer, or wisdom for cleric, ranger and druid.)