Depending on condition of 1/2 cut..
ie - kms, condition of parts, if all parts are there etc..
if its jdm or not
where your gettin it from - ie wrecker or just a guy who wants to get rid of his post-accident car..

but id asume anywhere from 3-5k depending... they are alot rarer than the b16s from my experience..

Whats involved?
Move everything from the 1/2 cut to the CXi engine bay =D

nah seriously, u will need to firstly get new engine mounts for the b18, you wont be able to use ur exisiting engine mounts..
Then you will pretty much do wat i said above.. chuck everything into the engine bay...
You will need to change the wiring loom and rewire the engine to the ECU (make sure this is with the 1/2 cut)
You will alos need to change the whole Gearbox, gearlinkage, clutch and pressure plate all from d-series to b-series..

Make sure all these parts are with 1/2 cut cuz if anything is missing it can be very hard and expensive to find at wreckers or aftermarket...