In all seriousness, the most expensive part would probably be the Differential Wiper system. They are a common thing to go on that model and are insanely expensive. My buddy works at BMW and HIS cost is 3200$ for the parts.
91 318is - M50NV (gone)
02 WRB Subaru WRX EJ207 forged, 320 AWHP stock turbo.
85 - 325