Define cheap?
Define test? I mean I always do a test cut on my real materials to be sure I know what settings to use… are you testing like wanting to do a fullsize cut to see if you like it?
If so, I’d say baltic birch ply is a good test material, tends to be really inexpensive (like $1 per full GF sheet, when purchased locally).
Matboard from art stores would be good, especially if you buy the gross colors nobody wants when they’re on sale.
12x12 cardstock is easily gotten from amazon, or even better 11x17 cover stock. Also inexpensive.
All comes down to exactly what of the design you’re testing. Sometimes you just want to test a tiny portion, so I’ll crop it down and just try out that detail, like when testing an inlay:
Does that answer your question?