Mine are very roughly because I don't like to think about it too much bedding, I buy large bales of straw for £20 each, which last about 3 weeks each in the winter, nothing in the summer when the horse is out 24/7.
Hay costs about £10 a week in winter, next to nothing in summer. Feed, is about £40 a month, but I have a big horse, who needs a lot of feeding, it would obviously be much much less for a pony.
Insurance is £45 a month, shoes are £70 every 6 weeks. Wormers are about £7 to £20 each, depending on which one they need, that is about 4 times a year. Vet for teeth, jabs etc, once a year. I can't remember how much that is, but it is always more than I think it should be. I think it was £80 for just jabs last time.
Then there are new rugs, or having rugs cleaned as and when they need it, new bits of tack, new boots, hats etc for growing children, as well as the cost of lessons.
It is not cheap, but it is well worth it if you can. It is the only thing that keeps me sane!