According to a handful of readily available and casually discovered internet sources, it would cost around $62 Trillion to create a gram of Antimatter; but the good news is that apparently, with enough effort, it could - with our technological knowledge - be done.
Earth assets according to another source are worth roughly $140 Trillion. Thus, in hypothesis, focus of less than 50% of global assets could pay for said antimatter.
For a moment, let us assume that we achieved world peace, poured 50% of the world's fiscal reserves into a massive new space program, created a gram of antimatter, and are now going to decide how to use it.
Howstuffworks.com suggests that one millionth of a gram could sufficiently fuel a year long Mars mission; let us assume that this is correct, and that we are planning to pour that entire energy arsenal into one massive ship-building project.
What would it look like? We're talking about enough power to fuel one million trips to Mars without refuelling; clearly more than enough to create a fully navigable ship capable of supporting a crew for an extended duration and travelling outside our solar system.
You're asking all the wrong questions, methinks. The political and technological implications of the stuff would be staggering.
Leaving out the politics, I suggest this: one gram of antimatter--say, a penny-sized chunk of the stuff in a magnetic bottle--would be the single most valuable object in the entire solar system. It would be like "the Galaxy" in that stupid Men In Black movie, only with the unfortunate tendency to level entire cities when jostled.
Using the whole gram on any one application would be kind of stupid, since that's more energy than any system could realistically harness. But a government that obtained it would have to develop a system to distribute nanograms of antimatter to different end users in extremely robust containers under extremely tight security, and even then, you're using the antimatter literally a few hundred atoms at a time for most energy applications. Indeed, even in a starship, it's likely that antimatter is consumed in trickles of particles, zapped into their necessary components in quantities no thicker than the electron beam from your TV's cathode ray tube. That's alot of particles and alot of energy, but not alot of MASS.
A more interesting question would be "What if SOMEBODY on Earth had a gram of antimatter and nobody knew for sure who or where?" THAT would be the premise of one hell of a sci fi novel.