If you are thinking of putting the horse on working livery- which I do think would be a good potential solution-then your costs will be lower than on fully livery.
On average, working livery will cost between £40-£70 a week, depending on what is included and how many hours you are willing to allow the horse to be used for. This would usually include the cost of bedding, feed and most horse care. Specialist feed and supplements probably wouldn't be included in this price.
On top of this, you would usually have to pay for a set of shoes around once every 6 weeks at around £60 a set.
Insurance +medical costs will usually be on top of this and may be slightly more expensive due to the horse being on working livery. £60 a month might be a reasonable estimate for this.
So, say your baseline cost per month is around £300-£400.
Lessons with an instructor would probably be on top of this- and it would be a good idea to keep up with this at least for the first few years.
You'd also usually be expected to supply all the tack/equipment the horse needed and replace it if this was needed.
For tack, the price is hugely variable, but I would expect you to be able to buy a bridle for £100+, a saddle for £500+, rugs for £50-100 each (how many you need will depend on the pony). The saddle probably wouldn't include a numnah, stirrups or a girth, so I would budget another £70 or so at least for these- and they would possibly be more likely to get lost or damaged while on working livery (and a certain style of stirrup might be insisted upon for safety reasons).
That's still well within your price range, though.
I think the thing with working livery is to go into it making very clear what your expectations are, including the number of hours you want the horse to be used for, the type of riders you're willing to allow to ride the horse, and the level of work you'd want the horse to be doing. Having a whole weekend day when the horse isn't used for lessons might increase the price for you, or might be a sticking point for some stables.
You'd also need to think about and discuss what would happen if the horse wasn't able to work due to lameness or injury, or what might happen if the riding school decided the horse wasn't suitable for use in lessons any longer.
Ideally, at the stage of negotiating a contract, it would be great to have someone horsey but not involved with the riding school to be on your side during discussions.