1972 Columbia Tourist Expert III, Columbia Roadster
Well how much work you put it into it is going to depend on how much you want to do with it. If you're going to keep it then I guess go as far as you need to go to be happy with it.
If you're going to sell it - you'd probably be lucky to get $200 or so for a nice one, so you probably don't want to go all out. If it were mine I'd replace the tires if they're old/cracked/worn out, repack the bearings with new grease, oil the cables and adjust the brakes (if it has any - some only had coaster brakes) and pretty much leave the rest as is other than cleaning off any dirt or grime/obvious dirtyness.
If you start going all out replacing the chains, seats, cables, etc. you'll easily spend the worth of the bike.