Whether it costs a depends on what you want to do.
If you want a decent vintage bike that you an use for fun you
can pick up a bargain.
If you want to restore a really collectible bike you can spend
I'm leaning more towards the former at the moment. I'm rebuilding an
old bike for the Retro Ronde Van Vlaanderen. The budget is something like this:
Original bike: €50
Replacement wheels: €30
New cables, bar tape etc: €30
Paint for retouch: €15
I think that's all I spent, but this isn't a rare or collectible bike and most of
the parts were reusable, only the wheels really needed replacing.
This site has a LOT of information about restoring old bikes:
http://www.mytenspeeds.com/My_TenSpeeds ... UCTION.htm
It's useful, but possibly also a sign of how obsessive you can get if you're not careful