If your talking about a USDM STI swap. (and just the engine swap, nothing else).
Motor online ranges from $3500 to $6000 for the complete engine/wiring/ecu/turbo/intercooler.
Transmission rebuild, plus heavy duty clutch $1000 to $1500. If you go with the STI 6 speed and DCCD I think I've seen these in the $4500 range.
As for difficulty. The motor will mechanically connect just fine, it's the spaghetti mess of wiring and some other minor fabrication that is involved.
UTCiv and I are the only BD legacies I know of with this engine setup. He can comment on the difficulty of the swap. Or you can search for his build/swap thread for more information.
My wife was nice enough to let me buy all the parts. If I chose to work on the car instead of my project house, I'm not sure I'd be alive right now, so I paid to have it done.