My cam chain stared rattling somewhere between 50,000 and 60,000 miles.
Never got round to doing anything about it.
Bikes now done 82,000 and it's still rattling of course, but it hasn't failed.
As regards cost, it depends on whether you fit an endless chain (very expensive), or a soft link one (much cheaper).
If memory serves I think I was quoted around £500 for an endless one because the engines got to come out, and £150 for a soft link one, (both prices were for labour only and were ex v.a.t.) but that was ages ago.