Most of my work was gates and railings etc, I simply priced the job up and then added 10% to cover gas, wire and electric, that was easy because I was just 'small time' working from a home workshop with no rent or rates to consider.
I find most people are not bothered about £5 for consumables because at the end of the day you have still had to buy them and I spend quite an amount on my consumables van each month.
as far as welding goes I stick a £5 but if I am using dissimilars i charge them out at a £1 a rod with them costing £100 box.
I put a nominal sum on a job for consumables - I've never worked out how much it costs on consumables say per tonne of steel as it will vary wildly anyway depending on how much welding their is, what the job is etc.
A job with say 8T of steel, I will put £300-00 on.
I can burn a box of rods in a day, full out - 10 hours.
I figger on £25 a box (6013 white box), so +2.50 on my hourly rate.
That figure will also cover bandsaw blades, drill bits, grinder blades, wire wheels when I'm not welding.
I make my own designs, to order - make one as a prototype to see how long it takes, then set a price for subsequent copies according to an hourly rate.
That works out as a good rule of thumb - for me!
I only use mild steel, however - no stainless, ally, or cast iron.
with a mig building sheds and stuff like that. i tend to do 15kg of wire in a day, 2 days to a bottel of gas usally have a new tip each 15kg roll of wire.
with arc for cast or other funky rods most people i know charge somehtign like £1 per rod. personaly i do very littel arc welding now just general botching don't charge for 6013 's on general stuff i proably don't get through a box in a year. if it's for a job if i opend a box id bill for it