I'd take the stance the pestering is bad behaviour and requires disciplinary action.
But a lot of the time this type of behaviour is partly down to inconsistency in parenting.
Are you certain if you say "no" you ALWAYS stick to it? With dd if I wasn't certain of my answer straight away I'd say I'd think about it or I needed to check the budget first or it might with this type of thing have been dependent on extra good behaviour from her. BUT the one thing I did from the start was use the word "no" and once I reached the point of saying "no" dd learnt from my actions that it NEVER changed to even a maybe let alone a yes.
But you have to be certain when you use "no" that or will remain a "no" before you use it.
I was a single parent most of dds childhood and never had much money and I was like you honest about that - telling her I simply couldn't afford it. Right from the age where it was worded "no more pennies"
As she got older I discussed with her in an age appropriate way what our bills were and how much things cost. I did my grocery and other shopping "in real life" then and discussed with her things like comparing prices, special offers etc
Have you discussed things like advertising with her?
From quite an early age I discussed with dd the purpose of adverts "to make you spend money on that companies products", that companies spent a lot of money on experts very good at designing persuasive adverts, we discussed what elements certain products employed to do this.
We discussed things like why it's cheaper to eat at home than eat out and the pros and cons of each.
And yes, we discussed that even though we had it pretty tough there were many others worse off than us in this country and overseas. Why this was, what could be done to change it, different types of poverty etc
Tell her exactly how much a big birthday party, redecorating her room etc will cost put it in terms she will "get" - perhaps x months/years of pocket money or even x months of your FULL wages, at her age you could even get her to price it up herself and work out herself how many months of your wage - how many hours of your labour it would cost.
What chores does she do? I'm not really a fan of linking all chores to pocket money. Some are and should be a normal part of being a part of a family. So stuff she does for herself - keeping room clean and tidy, including hoovering, changing bed and dusting/polishing, getting her school bag ready for the next day, sorting her laundry and putting clean clothes away - not pocket money worthy.
Doing chores in other parts of the house, Dishes, hoovering, mopping, other people's laundry, cooking, tidying, dusting/polishing - pocket money worthy.
Plus you have carrot - you need stick too.
My dd still occasionally went through pester periods in spite of all the other stuff I think they all do at some point but if she reacted badly to a "no" - grounded for a day! Kept it up? Another day. Grounded = straight home from school, no phone no tech time aside from homework which I monitored, dinner, brief leisure time, shower/bath and bed.
It's not too late but I do think you need to tackle this now before the teens hit.