Transmission Choice: 2007+ LGT Spec B 6MT, 2004, 2005 STI 6MT $2500
Rear Differential Choice: 2007+ LGT Spec B R180, 2004+ STI R180, <2004 Legacy or you can use a regeared stock R160 as well with stock LGT rear axles $250
Axles: Front - Stock LGT, Rear - 2007+ Spec B or custom Driveshaft Shop Axles $750
Etc: DCCD Controller for STI 6MT $350, 2005+ 2.5i LGT Auto Driveshaft$50-150 (depends on the junkyard, dealer etc), appropriate 6MT flywheel and clutch I picked up a stock STI flywheel for $25 I think, then a SPEC 3+ clutch package for $750 or so I think.
Add labor on top of that if you're not doing the job.
You'll still need the STI R180. It's not that its an R160 or an R180 but rather the gearing that is important. The STI and Spec B R180 are a 3.9 final drive, which is what is required for the STI swap, so you will not be able to use the outback's stock R180. You may however be able to use the axles, if you do the swap, you can take a picture of the axles and I'll be able to tell for you, I think there is a pic in Hammer's thread from my swap comparing the Spec axles to the stock LGTs.