The price depends upon the condition and where you buy it. Such bikes are sometimes found at Goodwill or at yard sales for $25. But, if you buy from a good LBS or a reputable E-Bay dealer who certifiy that the bike is in "good" riding condition, the price might be more like $125 to $200.
And, often, you will need to budget for new tires, new saddle, new pedals, a tune-up...a really nice 1985 bike restored to "excellent" riding condition can end up costing $300 to $400 by the time you are done.
Given that NEW 2005 road bikes can be found (especially during "special sales") for under $600, it may not seem sensible to "invest" $400 in a 1985 bike. Yet, if you find one with some attractive features (Reynolds 531 frame and fork, top of the line Campy or Sun Tour components, classic touring geometry) maybe spending $400 can make sense.
It seems as if most "fans" of vintage bikes have a good story about a $25 "find"...Not me, but tomorrow is another day.