(very very very extremely rough guide) as you say no particualr car - make a trasmission tunnel, rear beam mount from a RWD car and diff mount etc. engine mounts for a longtidudinal engine and gear box mounts.

for a mk3 escort i would cut the floor pan out and fit one from a sierra then its bolt on (well its alot of work but far less then swapping the floor) to put the sierra stuff back on

cost: alot, unless you love the car and thats what you want to do, or want to do something radicul with it

worth it: as above really, far easier to buy a rwd car that you like