The correct answer: It depends.
-Engine type
-Total time on airframe
-Cycles on airframe
-Are the engines included or leased?
-Inspection status ie recent heavy check or due for one?
-Damage history?
-AD compliance coming due or previously performed?
Reading between the lines I would guess the conversion cost is between $30M-$50M USD not including the airframe. Again reading between the lines for a GE powered -400 is around $80M-$110M USD.
Out the door, you should expect between $100M-$130M for a freshly converted -400BCF from Boeing.
FYI...I'm just a Pilot.
DBW