Strictly speaking, I don't believe that spell storing can be conferred on ammunition; or at least, that the benefit of doing so is iffy at best. According to the text of the ability, the wielder of the weapon gets to cast the spell stored as a free action at the time the weapon strikes. Of course, you're not wielding the weapon at the point when you hit your target...
If your DM does allow it to work, it has to be the most efficient method in terms of cost: spell storing is only a +1 property, so you get fifty for the price of a +2 weapon (assuming the arrows have to be enchanted to +1 first).
Considering there's no CL limit, that's a pretty sweet deal compared to a wand, not to mention the fact that you'll get multiple shots per round. On that note, if your DM does allow spell storing arrows, don't be terribly surprised if he or she insists on a standard action for one shot, just "because".
If it's not allowed then just treat the arrows as generic magical items: single use, use-activated. Spell Level x Caster Level x 50gp a pop (base price).