For the look of vintage plaster, coat a urethane ceiling medallion with joint compound that's been thinned with water until it drips. Once dry, the medallion is ready to install.
$30 to $50. Medallions in various patterns and sizes, from about $15 each; architecturaldepot.com
. Dap Fast 'N Final Lightweight Spackling, $7 for a 16-ounce tub; amazon.com