I highly doubt we will ever be able to achieve interstellar travel for 500B or less, not with our current technology, maybe in 100+ years, but when coming up with a number we need to also take into account all the tech we need between now and launch and the costs of those technologies, not to mention we'd need a wormhole, either one we create, or that someone else creates for us, and since we don't know if we're alone or not in the universe, the only assumption is we'd need to create that ourselves, and I really have no clue how much a Wormhole would cost to create, but I'm gonna guess at least 20 Trillion USD, if it's even possible at all...
$200B estimate for the fictional "The Martian" mars trip sounds low to me, though it does bring into perspective what could have been achieved with the trillions spent blowing stuff up here on earth.