I pretty much do base it on the metal used and the quality of the stones if any. If it's gold and they use junk stones I don't qualify it as fine. But if it's gold or plat and has nice quality stones, I consider it fine. I don't consider silver fine regardless of stone quality.
I consider fine jewelry to be made from silver, gold, platinum with or without real gemstones.
Costume to me would be plated metals and rhinestones/cz.