hi mate, looking to do this soon also. The cosworth YB utilises the same 205 block as the injection pinto engine, therefore dependant on what engine you have in there at the moment the mounts may or may not need changing. One expense will be an atlas axle with ideally a zf motorsport diff which are expensive but apparently without it the cossie power will munch through diff after diff... to put a bog standard setup in u would be looking at, 2000 axle and diff, 1000 good engine and ancilliaries , i'm going for a t5 box which requires a bigger transmission tunnel than that of the mk2 escort. The easy option is to use the type 9 box. You should pick up a standard t5 for under 300 if you look around, however i am not sure how much power these will withstand before requiring uprated internals.
for a turbocharged YB you will need a cossie wiring loom, these tend to go for between 100-200 pounds in my experience, more with ECU.

To fit the engine the sump or crossmember needs to be adjusted as it does foul, however there are afew ways round this, one being a winged sump and there are other methods which i forget tbh :s

If the mk2 is non injection at the moment then the fuel system will need to be sorted, tank with swirl pot, pump, filter, return pipe etc which is extra expense.

Add to these costs the intercooler, radiator, and any modifications for example brakes, suspension etc and it is not the cheapest job in the world but it isnt expensive and it is awesome

hope this is some help, i have more than likely forgot a lot of things but i think i hit the key points. Good luck!