I don't know the rules, but I would go with a more realistic standpoint and charge the same amount as a Spear, since its pretty much what ballista bolts were. For other siege ammo, I would ballpark it by using the price increase for large and exotic armors, which is something like a x4 price increase?
Looking at the SRD, it says that enchanting the siege weapons themselves cost 2x as much as normal, so I would retract my earlier statement and say that the ammo would probably cost 2x, so 130gp (10 base + 2x 60 special material cost)
