Depends entirely on the Mercier. The classic models came in three levels, 100, 200, 300, in ascending order of quality. I would look for frame tubing stickers-- Reynolds 531, Vitus, and Durifort are likely on better French rides. I'd probably drop 50$ on any level French Mercier, but that's me-- I like old Frenchies. USA z's hints as to quality manufacture are well put, esp. about the crank, but some french manufacturers did not always use integral derailleur drops on high end bikes; Gitane and Jeunet come to mind, not sure about Mercier. You might look for the words "Simplex" engraved on the drops in the case there is no hanger; most hangerless hi end frenchies I've seen have a plain simplex drop.

Be aware that a classic Mercier may very well be French threaded, which raises possible compatibility issues with modern parts. They can be hard to upgrade and if either the headset or bottom bracket is shot, those parts can be difficult but not impossible to find. In addition, many shops border on incompetent or at best mystified when dealing with French stuff, so ask questions at the shop when you take it (assuming you don't do your own work).

I believe one of Lance's first real bikes was a Mercier.