You need to know if your full livery includes all hard feed, hay and bedding, it would be very very cheap (round here anyhow, west yorkshire) at that price with feed hay and bedding included. Also, do they poo pick the fields or is that extra.
Realistically you are looking at a farrier visit every 6-8 weeks, worst case scenario you have one with crap feet that needs a full set of shoes every 6weeks, thats £55 per set of shoes. Best scenario you have native pony with feet like iron which needs a trim every couple of months at £15 a trim.
You will need to have teeth checked twice a year, this tends to be cheaper the more horses the dentist sees, so if you all club together, £30 per visit should be enough.
Then we move onto back specialists and saddle fitters, some ponies never have a problem with their saddle and never have a sore back, but most do, so you can expect at least one visit from back specialist (£60 ish) and your saddle may need tweaking but this comes under tack repair below. ,
Vets, you have a yearly vaccination booster costing around £60 and a wormer every 3 months at £15-£20
You can also expect a couple of adhoc vet visits in a year, they inevitably vandalise themselves in the field or while out riding, or may have a colic or some other random illness. I had a call out last week, course of bute, course of antibiotics, a light sedation so the vet didn't get his head kicked in, and sn exam, cost me £125.
You will also need horse insurance which depends what level of cover you need and the value of the horse.
Finally allow a couple of hundred quid for new tack, rugs, repairs and fly spray, first aid top ups etc. This is on top of the initial buying of tack and rugs.
It's an expensive lark this horse ownership thing!