May be worth looking at what you would need to get to 450 HP
http://www.racedynamix.co.uk/remap/subaru-remapping/typical-upgrade-path-newage-2-0-sti/
The above link gives a good rule of thumb
Just weather the cost saving is worth the difference in breaks, 6 speed transmission, some stronger parts, bigger top mount, bigger turbo and so on.
but on the plus STI parts are normally avaliable if you find someone breaking up an STI, or if you could find yourself a doner STI ?
All depends on what you can get for what price