It has been said already but I'll repeat it. Restore can mean a lot of things. If you are looking to make a bike clean and rideable, that isn't really restoring it. It's making the bike functional, rideable, and safe. Restoring it would include upgrades of components to whatever matches the culture and quality level of the bike, like putting French (or Suntour) derailleurs on a French frame. A high-end frame deserves really good components, especially if your rides may be long and hard. A commuter or cross-campus bike needn't have that level of components, but it would be restoration if you replaced junk components with culturally and chronologically correct components. Similarly, a low grade frame isn't worth the expense of high-end components.

How do you tell if it is worth it? That depends on how you intend to ride it and what you are starting with. Functionality doesn't have to be expensive, so that's a limiting factor. It isn't worth spending $500 on components and wheels just for 2-mile runs across campus or across town. But $20 is small change for entry into the game.

I hope this makes sense.