Our local DIY livery charges £120 per month, and we are supposedly in one of the most expensive parts of the country.
I've saved a lot of money this year by switching from small bales of hay to large bales - they are more of a pain to move (I get them delivered as close to the barn as I can and then cut the strings and move them slice by slice inside.) It's a pain, but I reckon it's saved me about £340 this winter.
I also started buying my bedding in bulk. My horses are on rubber matting with a wood pellet bedding. You can buy it buy the ton for about half the price if you buy it from a fuel website (you don't pay VAT on fuel, and you probably have a solid fuel boiler, don't you? ;) )
If you are spending £10 per month each on worming, teeth and vaccinations, you are having the wool pulled over your eyes. You shouldn't be worming more than quarterly (get a decent schedule from your vets, many equine vets will have an information page on their website with a decent schedule on) and Pramox (one of the most expensive wormers) doesn't cost more than £18 a tube. Our vets' do a free call out day if you are prepared to accept the appointment time they give you. Sadly for me, the free call out day is the same day our hunt meet, so I can't use it through the winter. I try to marry up teeth and vaccinations at the same time, and pay about £80 per year for each horse/pony.
Wrt hard feed, basically a horse should be fed in accordance with the amount of work that it is doing. You don't need a nutrionist, as has been suggested - call each of the big company helplines, and asks their advice. Be honest about the amount of work your horse is doing - 5 steady hacks a week is not hard work. My horse is currently hunting twice a week, 4-5 hrs work including fast work and hills each time, plus a 2-3 days a week of an hour very brisk hacking. He is being fed a considerable amount of conditioning and slow energy release feed to make sure he comes through the season in good condition and has the energy to keep going all day without being a fruit loop! plenty of fibre and oil based feeds, they are surprisingly cheap and he looks great. He doesn't get any balancer, vitamins, additives, etc - firstly he picks the bits out, secondly the feed companies that make stuff other than just balancer will tell you that you should only need balancer if your horse's diet is lacking in something. Be sure you know what it is you are trying to supplement in your horse's diet rather than just slinging some price pellets in because if it costs that much it must be doing them some good!
I actually totted up costs not too long ago as someone asked me for a breakdown with a view to buying a 14.2hh Connemara. This is the breakdown I gave:
Bedding £2.50/bag. Approx 70 bags a yr. You need rubber mats to start with with this bedding.
Hay £40/large bale.. Approx 7 bales a yr
Hard feed costs me about £40/month.
Shoes £60/ set, every 5-7 weeks.
Worming, teeth, jabs, about £150/yr.
Insurance - I don't insure the horse, I put money away for vets fees (£20/month) and have BHS Gold membership (£65/year) which covers me third party. Alternatively you can insure the horse for minimal loss value for the veterinary and third party fees.
I should add the reason I don't insure the horses is because the 'valuable' young one had a very serious illness as a 5yo that effectively makes him prohibitively expensive to insure, the lovely pony we are keeping forever is 'aged' now, and the third one is nearly sold anyway!
We worked it out to be about £150 covers the basics each month, plus livery - £120-ish if you are there morning and night, up to £5 per day extra if you need help. Lessons should be about £30 per lesson if you get a local instructor, prepare to pay more if they have to travel a way to get to you. Tack, vet visits/insurance excess, competitions, physio, new rugs (for the rug destroyers out there! But don't get sucked into buying millions of cheap crap ones, I have 2 Premier Equine outers and 3 liners per horse, and they live in and out in those,) these things are all extras. You can pick up equipment cheap at horsey car boots and ebay, but make sure you know what you are looking for.
Sorry if this is long winded, but as you can see, there are many ways of skinning this particular cat!