Don't know what it costs, but I would still say that stainless steel is too expensive, galvanized is the most durable in relation to price, and wood is the nicest. An external staircase I would build in pressure-treated lumber; if you can attach the entire structure to the wall, it should last a very long time, but even a structure resting on the ground will last if it is pressure-treated long enough so that it won't be a problem. There are different classes of pressure-treated wood that I don't know by heart, but it should be easy to check at the hardware store. But wood always requires painting sooner or later, usually sooner when it's outdoors, so the maintenance is more work on one like that.