another vote for cheap paper plates...., BUT..!! another cheap target i discovered, is get a roll of that packing paper that you get in shipped items, it can be bought from moving companys, i took a large sheet of plastic (i got mine from a popular video store) like you see in store windows advertising stuff, cut out a stencil or two or more, lay it on the paper & use any color spray paint you desire.
if you want "Shoot'n C" targets get a roll of freezer paper, the paper that has a slick plastic like side, use a stencil & spray paint the slick side with your favorite color , these are not as good as the real thing, but through a scope you can see the white inside the colored area, as for cost.., i figured i was spending about 3 to 5 cents a target !!