Typically, a fully built motor consists of two separate things: a built "short block", which consists of your pistons, connecting rods, bearings, crankshaft, and possibly ductile sleeves depending on power level goals. When someone offers a fully built motor for sale or offering to build a "fully built motor", it also consists of building the cylinder heads of the motor, which is essentially the "top half" of a motor. The cylinder heads include your valves, camshafts, valve guides, rocker arms, etc.
Since the amount of work involved can vary, cost can have an extremely large range. Built motors cost wise DO NOT include any intake, exhaust, ignition, or fuel system parts, as well as turbos.