Cost depends on what model/year sti engine you use.
Its gunna cost atleast 6K, thats if you do all the work yourself, apart from the loom splice.
probs budget double what i have quoted above, and thats using an older sti engine, from a gc8.
A halfcut is the best best way to go, that way you have most of the parts required.
Cheers, i have a budget of about 10k so i could push for a bit more if needed, i found a whole car which is a 96 sti gc8 for 4k so ill buy that and do the conversion with that. do you need a new tail shaft and drive shaft or does everything fit well??