Maybe 60-70p per week on juice (unflavoured, not massive nic strength, 60ml lasts me a couple of weeks or so - or so being 1.5 weeks to 3 weeks depending on my mood). Oh, plus one or two flavoured shortfills that I dilute at least 10:1 and only use a little per week.
Coils, pennies - I've nearly finished my first pack of cotton after just over a year, a 10m spool of wire lasted me maybe 10 months (including messing around with twisting multiples but excluding maybe 20 coils made with different wire).
I do go through a few spryte coils - £5 for 5 coils which last possibly 3-4 weeks each depending on what I pick up each time. So that's 30p/week on top of the rest.
Altogether, running costs (excluding hardware purchases) I'd estimate at something like £1.50/week.
Plus electricity to charge the batteries
Then the wife - a very quick calc resulted in me thinking it's about £4/week for juice (majority mixed by me, a small amount of 10ml bottles) and maybe another quid or so weekly on coils.
So altogether, for two of us, £6.50/week or thereabouts.