A shitload I would think... Not sure how big the ramp is though.. LOL..
But try putting shit underneath it first.. Like tyres, things like that, then the dirt on top or it would end up being costly..
Also put a sign out front or your house or a main road saying drop off dirt for free, shit like that, companies will be happy to so they don't have to pay to get the shit dumped..
good luck with it mate.