you have to accept that you will not win every job, but that you must price to do a good job using materials of a quality that you are happy to endorse, (ie you will be dealing with faulty/waranty issues)
Personally I have a costs list for materials, eg socket, back box, plugs and screws = £5 per point, switch point =£5 shower pull cord £15, oven point £25 etc etc
these are the cost for the materials, so i'm not then adding screws etc etc to the job, and it keeps it simple for me.
I then get an upto date price on the cable required, and simply use my many years of experience and a finger in the wind to guess how much cable i actually need, usually for a three bed semi i'd expect to use 150m each of 1.0, and 2.5 and i'd allow 100 of 3c+e @ 1.0mm, then whatever for shower, cooker, tv, telephone data etc etc etc.
Then simply add the cost of labour, again an experiance/wind finger/ guess type thing.
sometimes you get lucky and you'll a) get the job and b) make money on it, sometimes you'll get the job then get messed about, and break even.
for few ideas, recently we've done a 3 bed bungalow £3500, a two bed semi £3700, these are both on the basis of 2 weeks labour plus materials, no VAT.
it's not unlikely that even if they haven't got a cheaper quote they will tell you they have, be prepared and stick fast, I can get 2g sockets for under 50p each but there's not a chance i'd like to use them, it's hard to quote against others without a spec.
good luck