A Bow is best for farming cause you don't need to run from one mob to the other. You only need to find a good spot and then just kill everything that moves.
So if you have the money then a lvl8 SOM/SOSUN, lvl 16 SOS/SOM/SOSUN Bow or if you don't have enough money you can also use a +5 (or more).
Well as for the prices (these are prices on Red See for a lvl 16 Bow):
SOS = 6mil+
SOM = 20mil+
SOSUN = 65mil+
Well occasionaly you can get them for lower prices. But that is very rare (mostly a mistake or a new player that doesn't know it's value).