You can make tombstones cheaply, like others have suggested. I made a bunch for my yard using sheets of styrofoam insulation from the home improvement store. It's cheap.
I cut the shapes out of the styrofoam using a serrated knife to saw through it (it's messy so be warned). Then I painted them using faux finishing techniques and staked them in the yard using wooden dowels. You need to sharpen the ends of the dowels with a knife first.
Then I put moss around the bases, and added some fake cobwebs.
Here are some pictures: