so many flaws,
It hurts, i can see so much potential for this. here is my list of cons (I'm sorry in advance):
your rating system is off: easy = hard, normal = nightmare, hard = internal bleeding
I noticed a couple weapons would miss, mainly the skybeam.
there should be a better choice of weapon for the "both" category, or a cheaper and less effective "air" weapon.
why am I spending 225 gold to upgrade range (at the max) one unit farther?
the upgrade system overall could use an overhaul. I'd suggest a 1-10 level rating for each of the three categories, and a set price for each level (weapons with an already high stat would cost more to make it even more damaging)