There's no such thing as a 9mm Desert Eagle.
None.
You have to be more specific. Looking at the "Magazine" or "clip" can give you a good idea on how much ammo a weapon can hold. Also the caliber size of the ammo is a factor to how much ammo is in a "Magazine/clip".
100-1000 usd depending on specifics
Pretty much a 1:1 copy
usually in a A-10 II carries 1,200 ammo, which at maximum R.P.M. can be expended in 17seconds
"Magnum" refers to the cartridge, not the firearm. The Desert Eagle fires Magnum cartridges. What you're probably asking is the difference between a revolver chambered for Magnum cartridges, and a Desert Eagle semi automatic pistol. Of the two, the revolver would typically be much more practical.
Probably around 60 to 70 dollars
2000-2500 USD or so
i saw a gold plated one going for 3k at a store
An eagle can typically carry prey that is up to half of its own body weight while flying.
Aresenal is the stockpile of weapons or a building to hold weapons, pretty much like an ammo cache, but for weapons.