A shitload I would think... Not sure how big the ramp is though.. LOL..
But try putting **** 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, **** like that, companies will be happy to so they don't have to pay to get the **** dumped..
good luck with it mate.