When you buy weapons and vehicles, you will see a value for upkeep.
Upkeep is the amount of money that is taken away from your hourly income.
So, if you were making $10,000 an hour and you bought a Vehicle with an Upkeep of $1,000, your hourly income would be $9,000.
Weapons (and vehicles) make your attack/defense scores higher, increasing your chances of winning against your opponents.
Weapons in this game come in sets of 100. Each member of your squad can hold 100 weapons.
Keep in mind that when buying weapons, the attack/defense score is for all 100 weapons, not just 1 weapon. When you buy 100 weapons with an attack score of 5, your attack score can increase by 5, not 500.