To buy gold while visiting the vendor, tap the weapons icon on the right side of the screen, then press the $ icon at the bottom of the pop-up window.
To buy gold in game, hit the pause button (top right), press the Armory or Skills icons, attempt to upgrade a weapon/item and press the Buy Gold button. Available prices are:
2,500 Gold Pieces- $0.99
10,000 Gold Pieces- $2.99
30,000 Gold Pieces- $4.99
100,000 Gold Pieces- $9.99