Not quite sure what you mean by your costings - do you intend on paying someone £10 per hour to work for you?
I would not think about how much you want to be paid for working and how much you want to give your mate - think of it as a job where you have to advertise for staff you don't know. Average pay rate would be approx £6 per hour add on your materials, NI (although you shouldn't have any with the low hours still put it on as when you grow someone full time may take the position), Holiday pay, P&EL insurance costs, machinery, uniform, admin and anything else you can think of as an overhead then add your profit on and that will bring you out at your quote.
Fox