"Probably, stupid question: where does one buy ground cheaply?
And I mean just ground, not soil. "


If you need one load of soil to fix a yard grading problem, it is
typically screened topsoil that you want. You want 6+ inchs of good
topsoil to grow grass or support plant beds, etc. and that's what needs
to be on top. Only if it's a big deep area that would require more
than a single load, that you'd go with fill material first, then finish
it off with topsoil.