For common, small gauge wire/cable I use a per foot method most of the time since I'm cutting off of a stocked coil. If I know I'll need the majority of a standard sized roll of a size I don't keep on my truck (ex. 125' 6/3 NM) I'll take the highest price I find and add my standard materials mark-up to it.
For 800' of 4/0 I'd take into consideration whether I had guys delivering it vs. the supplier, whether it was pre-cut pieces or my guys were cutting the runs on site for T&M, etc. The less work I'm giving my guys at the site, the higher I'd raise my markup on the wire as a "convenience" charge, plus, you'll most likely be paying a higher price for cut pieces delivered, too. For 800' I'd still probably price it based on the highest available price plus mark-up. I would also take into consideration the ratio of the mark-up to the total job price. If your 8k number is 10% or 40% or 80% of the overall job price how will it make a difference in what you achieve over the long term with that customer?