Hey man great to see your getting into it and nowhere better to start than with some leo's they are extremely easy to care for and as is the way with most reptiles the biggest expense is the initial setup cost, I build most of my vivariums so it is much cheaper obviously.
I would suggest getting a wooden viv and avoiding these "starter kits" as they often don't actually have what you need nor at the right price. Shop around and look at some care sheets try this one Leopard gecko care sheet
It always depends on the age of the bugs and you could always breed the crickets etc yourself. I personally feed mine mealworms 9/10 and occasionally put some crickets in, i find that mealworms in a dish are much easier for the geckos to find and get the nourishment they need. as always supplementation and gut loading is the big one.
to your question i would say prices your looking at
Setup: £50-££££££ maybe even free
Feeding: £2 (depends on the age but i buy a tub of worms every week for my two geckos and they plump up nicely)
Maintainance: £4 every month or so for a bottle of disinfectant
Watching your gecko lick its lips and smile: PRICELESS
Best of luck to you dude