I buy a 2 inch thick sheet of syrofoam from Michael's and cut them the sizes I need... I glue them together to make 4 inch thick... I cover them with the sealing type of saran wrap, put 2 layers of icing on them, let that dry for a bit then play with my fondant on it .. when I'm finished, I just reach under the bottom and peel the saran off and toss it all in the trash can ..
I believe at Michael's here, it was $7 for a sheet of the syrofoam .. I made an 8" square and an 8" round dummy ... I had enough left over for half of another size so it wasn't too expensive to do.