Depends on what you want to use it for. Technically, it could be a masterwork tool if you can describe how it is used and for what skill. Or, it would not be a stretch to buy a set of clothing like hot weather, whatever and say it includes a black cloth face cover. Most of the clothing descriptions are not completely inclusive. IMO, the important thing is what you want to do with it and how it impacts the game. The more mechanical benefit you want to get, the more a GM is going to expect you paid for the item appropriately and it can be referenced from source material. If it simply for cosmetic reasons, buy the cheapest mask listed. There are no rules for equipment wearing out, so it'll hold up just fine.