The short answer: 2000gp. That's spell level times caster level times 2kgp for a continuous effect.
However, it get's a bit iffy with spells that can be expended (like protection from arrows). The best bet is a command word item; that is relatively workable and understandable.
This is where the 'system' breaks down, and I'm not talking about the magic item creation system. The magic item system is simple and makes sense; it is internally consistent. While some people feel it is broken, or can produce broken items, I counter by saying that certain spells are broken. The magic item system shows just how broken they are. Flight, haste, invisibility, etc. are all under-leveled for what they do.
I don't think Sanctuary as a continuous item would be broken; the save DC is probably far to low to matter at higher levels, and who takes Spell Focus and Greater Spell Focus in Abjuration? It just isn't worth it (not that that would affect a magic item).
So, I would suggest that it simply be 2000gp, but you need to add some special circumstance ot the item to make up for the fact that the spell can normally be expended (by attacking, etc.).
-Fletch!