Given the technology to create and store antimatter in quantities exceeding a handful of atoms doesn't exist, it's a bit ridiculous to extrapolate a price per gram for it.
"Your printer has detected 3rd party antimatter. Please only refill with Genuine HP(tm) AntiMatter cartridges. Authorities have been notified and are enroute to your residence."
It's a subscription model for the artificial stuff. The natural version is dirt cheap. It's always the middle man with these modern services, I tell you.
The reason why it's called antimatter is because the polarity of the nucleus and electrons are reversed. There are also antineutrons that have a neutral charge. It all still has mass, but will obliterate upon contact with regular matter
There are also anti neutrons that have a neutral charge
Expanding onto this, it raises the question: how is a neutron different to an anti-neutron?
A neutron can be though of a particle composed of 2 down and 1 up quarks and lot of gluon's that keep everything together. The gluon is its own antiparticle, so the antineutron has 2 anti-down quarks, 1 anti-up quarks and gluons. This way it becomes a different particle despite also being of neutral charge.
Just wait until we start having it manufactured in a cheap labor market. The prices will plummet! It'll likely be mostly fake, but that's the price we pay for cheap antimatter.
This leads to a modified version of the uncertainty principle: either you have a banana and can know the size of something or you have positrons and are unable to measure size.