When hardly anybody makes stuff for the Starion anymore (or the 4G63 SOHC engine) and you're one of the few suppliers who do then you can charge what ever you want. Also lower volumes of items sold naturally mean higher prices to cover tooling and holding costs.
If you want low item costs with high availability of aftermarket parts then you need to swap chassis' and go the "Nissan" way.
Also if my memory serves me correct Stage 1 and Stage 2 aren't that much of an improvement, however someone more mechanically knowledgeable should advise on this one.
1988 Mitsubishi Starion EX Widebody
1987 Mitsubishi Starion EX
1991 Mitsubishi L200 Double Cab Sport
2010 Holden Colorado Ute