I will try and put this in order as though you are buying your first horse......
First you will have to think where you will keep him, depending on where you want to keep your horse and where you live will vary in price... Unless of course you are really lucky to have your own field and stables!
If you go to a livery yard (this is where you keep your horse on a yard with other horses, that is run by the owner of the yard or a manager). Livery is either do it your self, where you look after all aspects of your horses needs, part livery is where you arrange with the owner what you would require them to do eg: muck out your stable and/or turn your horse out on days agreed, also feeding your horse. Full livery is where the yard owner looks after all your horses needs. The price varies quite a lot depending on what other things that the yard offer like some have a indoor or outdoor school, some have horse walkers and even cross country courses. I would recommend that you do a lot of research in your area if livery is what is right for you and your horse
Some livery yards are also riding schools and have agreements where you pay so much for your livery and they use your horse in the school, this can cut costs down quite a bit on your livery charge, but remember other people that you don't know will be riding your horse
If you decide on livery there should be a written agreement/contract this enables you and the yard owner to know what is expected on both sides eg: worming programme, insurance, vaccinations etc. Also if you chose livery find out what turn out is available (turn out is when your horse goes into the field) some yards have limited turn out so your horse could spend a lot of time in his stable. If you decide to rent just a field, again the price varies it could be anything from £60 to over £100 depending on where you live. For me personally if I did not have a stable I would need a field shelter, for emergencies and also to give my horse shelter from the weather. It is worth thinking if your horse was ill and you did not have a stable or a field shelter ( you could easily convert you field shelter in to a temporary stable)where would you put your horse? My horses lots of times have been ill and needed to have stable rest. just something for you to think about.
If you decide to keep your horse with other horses all horses need to be on the same worming programme for it to work effectively ........ There is a separate page on my web site for worming