Find a couple of boarding facilities and ask about full-care board. That needs to include turn out, blanketing/unblanketing, holding for farrier and vet, etc if you can’t make it there. Some barns charge extra for those services. The cost of board can vary SO MUCH, it is hard to say. Where I am, that would be about $600/month give or take $100 either way. But traveling a few hours any direction, and it can vary from as low as $300 to as high as $1500. So - there you go, first question is board cost.
Shoeing - ask your trainer’s farrier what he charges for (1) basic shoes, (2) common speciality shoes. You will see the price varies depending on what the horse needs. Most horses need to be shod every 5 to 6 weeks. Farriers charge significantly different prices even within the same market - I pay $125 for a basic shoeing - in my area, farriers charge from $75 to $300 for a basic shoeing.
Supplements/maintenance - does the horse need its hocks injected, SI injected, is it on oral supplements, monthly injections, etc? A healthy horse may just need a pound of ration balancer every month (figure $25/month), or it may need more - if you add hock injections, Adequan or Pentosan, and a few orals, you can quickly hit $100 to $200/month.
Vet bills - all goes well, yay! Then you need shots and basic exam twice a year, and dental annually - about $350 (ranges quite a bit). But here is reality - something always happens. A strained tendon, an eye ulcer, stitches - and sometimes something major - like colic surgery. So - figure you insure your horse for major events, $1,000/year (varies by horses’ value, so could be more, could be less), plus an extra $1,000/year in regular vetwork, minor things. Or, skip the insurance and put away a set amount into a savings account every month for potential vet bills.
Tack - you will need a saddle, a bridle and bit(s), halter and lead, blanket and sheet, horse boots, grooming kit, buckets, clippers - and perhaps a basic vet kit. Get on line and figure that out, the saddle alone can range from $500 to $8,000. You MAY be able to buy a horse that comes with some tack, so there is that…
Excluding the tack and equipment, your costs could range from $800/month to $3,000/month.
Showing and lessons extra… If you are showing rated shows - then there are all the memberships for you and your horse. Then going to a show - do you stable, or haul in and out? One day, or multiple days? Stay in a hotel? Trainer coaching, or not? Rated or schooling shows?
So many variables! The horse alone - could be anywhere from $2,500 to $100,000 - or more! And don’t forget to factor in a pre-purchase exam - with xrays, that is going to be $1,000 to $2,500…