A very difficult question to answer; as previous posters have said. In my experience, restoring a steam locomotive to working order on a heritage line took 29 years and the best part of half a million pounds. Like most projects, it is always the vicious triangle of the "dog with 3 masters" You can have something restored quickly, you can have something restored very well or you can have something restored very cheaply: You cannot have all three. If you get 2 out of 3 that's brilliant!
In the case of the 9F, its a really big job but, in some respects, not too bad as its a BR standard locomotive so some spares, fittings and drawings etc are not too difficult to find. There is also some experience in restoring these locomotives. I'm not sure of the situation regarding diesels but the bodywork seems to take much time and effort so would be expensive. However, the results are usually excellent.
Skilled volunteer labour and a large volunteer support group are more valuable than diamonds when restoring these beasties. Witness the very swift rebuild of the "cut and paste" Hawksworth County and now, soon to be constructed, the "night owl" Churchward 47xx.
