All the money you have in your bank account plus more if you let it.
I have 2. 1 is retired though and costs a bloody fortune in pain relief and supplements that I’m not convinced work but am too scared to stop just in case 
Basics:
£30 a week each livery - stable, field, use of school, electric and water (we pay extra for school lights)
Bedding - they’re on shavings which is about £8 a bale. They had 5 bales each when we started their beds then we use about 3 bales a week between them - £12 each a week
Hay - £80 a month between them, so £40 a month each.
We’re in a huge hay shortage here so am mixing haylage in to make the hay go further at £6 a bag and we use 3 a week, so £9 each a week.
Hard feed - varies depending on how much work the ridden one is in - £30 a month ish between them
Farrier - my retiree only has front shoes - £50 every 8 weeks. Our ridden pony is barefoot so just needs a trim every other farrier visit at £20.
Flu/tetanus - £60 each for flu, we don’t pay a call out as they do the yard all at once, but a call out is about £35. Can’t remember if we pay for the tetanus every other year.
Wormer - £10 each for a worm count, then about a tenner each for a wormer if needed
Dentist - £60 each, twice a year
Chiro - £60 each, twice a year
Saddler - £60 - we get it checked once a year, if it just needs reflocking or minor adjustments it’s included in the price.
Insurance - £60 a month for the ridden pony - we use Petplan as our vet does direct pay with them
£100 or so for water buckets/feed buckets/haynets/head collar, etc
Other stuff:
Rugs - second hand ones off FB selling pages do the job anywhere between £10 - £25.
Rug cleaning and re-proofing - I’ve just paid £65 for 7 rugs
My saddle pad addiction - less said about that the better tbh
Tack - depends what you want, DD’s saddle came with her pony and I got a shock when I looked it up, but you can pick them in fairly reasonably 2nd hand and have them fitted. We bought a micklem bridle recently as pony struggles with poll pressure which was about £150
Lessons - £25 for an hour, private lesson
Hat/boots - get a new, well fitted hat!
Then trailers, transport, etc, etc. Pony club? Show fees? Body protector, matchy-matchy shit,