Make sure you adjust all those numbers. A day for a weapon could be a huge overestimation if you're talking about a 2D game for iPHones - a week for a character could likewise be off, a month for a tile-set, etc. I would estimate higher for GUI. Factor in reworks as well.
I thought I had thrown in 2D Mobile in my original posting but I had left it out, that's why such a number was surprising to me because I was under the assumption that this was all for 2D. That's what I get for assuming though!
Also it might be worth to think about just buying models and/or graphics for some of your assets to reduce the costs. After all a lot of fantasy-weapons tend to look the same or at least very similar. The same goes for some GUI-elements such as health-bars. For enemies and especially bosses (and tilesets) you will still want to custom-model it, as this will make your game look more unique compared to other games of the genre.
Buying models and images will likely prevent you from applying a very unique art-style but the reduce in cost might well be worth it.
This is an option too, but the only problem I can see with this is if the "winning" artist's style does not match these generic pieces. The biggest thing I wish to avoid is a Frankenstein'd game with mismatching pieces. Granted it's easy to minimize the damage (Don't choose Sci-Fi pieces if the game is Fantasy) but it's something I hope to not even have to mess with.