Most people jump to cost as the first factor in their decision making, so we'll start there.
On a per pen basis, you can get a basic plastic pen for around $0.44 and a metal pen for about $1.22. The catch is that most companies only sell in bulk and insist on minimum order quantities. 100 pens is a fairly standard minimum order size.
The intended purpose of the pens also plays a role in the plastic vs metal pen decisions.
For example, pens make a good promotional object to send out in mailers. They're small, light and fit in most size envelopes. You can put your logo, business name and website address on it.
If you plan to send out 50-100 mailers to affluent leads, it's practical to pay the extra for economical metal pen. It creates a subliminal connection between your business and quality without putting a hurt on your wallet.
The ink can be specially made by the pen manufacturer. To be useful in a plastic ballpen, the ink must be slightly thick, slow drying in the reservoir, and free of particles. These characteristics ensure that the ink continues to flow to the paper without clogging the ball. When the ink is on the paper, rapid drying occurs via penetration and some evaporation. In an ink formulation, various pigments and dyes are used to provide the color. Other materials, such as lubricants, surfactants, thickeners, and preservatives, are also incorporated. These ingredients are typically dispersed in materials such as oleic acid, castor oil, or a sulfonamide plasticizer.