It is quite common to cost the job parts based on the list price (or what you would see in B&Q etc) and then whatever discount you get from the wholesale places is your markup.
Usually though that is not going to amount to much on cable, nor will it allow for partly-used reels, etc, that you end up with and might not be long enough to be usable in the near future on other jobs.
Also don't forget the "misc" stuff that adds up without you realising it, so clips, label tape, screws, glands, grommets, wagos, etc. If you can look back over, say, a year's worth of jobs and a year's worth of out-goings, you should get an idea of the non-obvious expenses that you need somehow to assign to each job's costing.
Usually though that is not going to amount to much on cable, nor will it allow for partly-used reels, etc, that you end up with and might not be long enough to be usable in the near future on other jobs.
Also don't forget the "misc" stuff that adds up without you realising it, so clips, label tape, screws, glands, grommets, wagos, etc. If you can look back over, say, a year's worth of jobs and a year's worth of out-goings, you should get an idea of the non-obvious expenses that you need somehow to assign to each job's costing.