Typically the cars that we have done C4 conversions in at our shop you will spend approx. 2500$ for everything brand new.
Fresh small bell housing C4 (we use transmission specialties) reverse manual valve body (trans brake optional) Red Alto clutches, new pump, billet servo, deep aluminum pan etc etc
Torque Converter (this should be custom ordered for your specific combo) Street cars need a converter with a sprag to keep the heat down. Spragless converters are great for track only cars but typically have too much slip for street duty. Most blower cars and nitrous cars we do use a 9" converter and N/A cars use a 10" converter. Over 500rwhp typically gets an 8" converter
Shifter
Lokar flexible braided dipstick
Transmission cooler
Braided hoses for the transmission cooler
Engine block plate ( I prefer Performance Automatic plate)
Flexplate
Flexplate bolts