There are a lot of variables that go into an overhaul such as what parts are re-used vs what parts are always new. Quality overhaul shops and "field" overhaulers are not equal in many regards. However for a Lycoming 4 Cylinder Aircraft Engine such as O-320 Series or O-360 Series you can expect to pay somewhere between $25,000 - $30,000  for a quality overhaul.