That's sort of like asking "how long is a rope?" You just have no way of knowing until you measure it.
A good engine man will take it apart, measure crankshaft journals, cylinder bores, etc. before determining how much work and how many parts are needed. Don't replace or remachine parts that don't need it.
I do expect the $8K number to be about right for a completely worn out engine. But, if the engine has had frequent oil changes and properly cared for, it could very well be that rings, bearings and a valve job along with timing chain components could get it back on the road in great shape. In that case it might only be $3 or $4K or so.