For $500 I'd slap together some fitting WordPress stuff to suit their subject matter and teach them about domain registry and hosting. Then I'd tell them to come back to me with more if they're interested in any specific features they're not interested in implementing themselves later.

$500 is worth a little bit of work and I can give them a product that's as aimless as their ambition for it. They certainly can't get more than they're asking for if they don't know what to ask for.