What primary weapon sticks out the least

The stat is based on how much money you've spent per kill.

For example, you buy a desert eagle in the pistol round for $800 and kill 2 enemies, your $/K stat will be $400.

The lower the number the better, it means you are more cost efficient as a player with the weapons/equipment that you buy.

A knife or a bomb. Unless you wish to shoot, then a Tec 9 or Five Seven

Terrorists

Knife - 250 Velocity
Bomb - 250 Velocity
Grenades - 245 Velocity
Tec 9 - 245 Velocity
Glock - 240 Velocity
PP Bizon - 240 Velocity
Desert Eagle - 230 Velocity
UMP - 230 Velocity
P90 - 230 Velocity
Mac 10 - 225 Velocity
SG556 - 220 Velocity
MP7 - 220 Velocity
Scout - 220 Velocity
Nova - 220 Velocity
AK47 - 215 Velocity
AWP - 200 Velocity
M249 - 195 Velocity

Counter Terrorists

Knife - 250 Velocity
Grenades - 245 Velocity
Five Seven - 245 Velocity
P2000 - 240 Velocity
Glock - 240 Velocity
PP Bizon - 240 Velocity
MP9 - 240 Velocity
Desert Eagle - 230 Velocity
UMP - 230 Velocity
P90 - 230 Velocity
M4A1 - 225 Velocity
Mac 10 - 225 Velocity
AUG - 220 Velocity
MP9 - 220 Velocity
Scout - 220 Velocity
Nova - 220 Velocity
AWP - 200 Velocity
M249 - 195 Velocity

From here