You cannot use polymer clay, must be natural. That price sounds about right for what I paid, as well. Nice thing is once you've purchased it, it'll last for 2-3 more tankfuls of MTS. The real tricky stuff to find is the dolomite and potash...highly advise contacting someone on the forum here to ship you some.
Again, once you have the clay/dolomite/potash...it's just $2 bags of topsoil.
I have done a 50, a 10, and now a 20 with MTS. Took about 1/2 a bag of topsoil to make enough MTS for the 20.
Have you done the drying process 3-4 times? You don't really need to sift until that point. Once you sift (I just pulled the screen out of my garage window), you'll get rid of any large particles.