In frame? Out of frame? Caterpillar, Cummins, Detroit? Cat is the most expensive. An overhaul can be just liners, rings, bearings. If injectors are replaced that will add to the cost. If the cylinder head(s) are replaced that will add to the cost. If a head is cracked that add to the cost. If the engine is pulled you will need to replace the clutch, engine mounts etc. If belts & hoses are replaced which they should be that will add to the cost. Any machine work is required on the block (cutting counter bores) will add to the cost also. Some brands & engine models usually require specific details.