people who have built tracks

Dirt is going to cost different all over the country. You will be surprised when you find that the cost is about 75% trucking and 25% for the load.
Also a variable is what type of dirt. I have found it cheaper to tell them I am not fussy about what they bring as long as it is relatively free of rocks and stones. I also make it clear it is for jumps so topsoil isn't needed. They usually are looking for places to haul loads with higher clay content (perfect for backyard tracks). Another money saver is to use some type of filler inside the jump- we use the pressure tanks from water well systems- they are free for the hauling at our local well installer.

Find someone thats going to dig a pond and get some from them or a constuction site where they are going to haul it away ?. I'm going to dig a pond for the dirt I need.