as others have stated, this will vary wildly based on location, discipline/breed, and amenities.
I often offer 1-2 stall board full-care at my private farm. I’m not a boarding facility, and often have no boarders (who are carefully screened to make sure they’re the right fit).
for my farm, full board includes:
12x12 fully matted stall with its own light, fly control mister, sealed motor fan.
stall is cleaned daily when horses are on regular t/o schedule, picked as required if horses are in more due to injury/weather/etc. Stalls are bedded with low dust pine shavings. Water buckets are dumped daily, scrubbed as required (e.g. daily or every other day all summer)
all day or all night turnout (depending on season) on grass in small group. Pastures are rotated for grass health, fertilized as required, treated for sandspurs and fire ants (regional issue), and we plant annual winter rye every fall. Fencing is wood 3-rail with all perimeter fences lined with no-climb wire to keep coyotes/neighborhood dogs, etc out. During periods of no/low grass (winter or droughts) we supplement with free choice hay by large bales. Groups are 3-5 horses matched for compatibility. Water troughs are scrubbed at least weekly. Pastures are picked at least 2x/week, and dragged, mowed, weedwhacked regularly as required.
We feed Triple Crown feeds, owner supplements as required, and all horses receive barn-paid feed-thru fly control supplement, Vit E/Sel, and mineral salt daily. The BIG cost is hay. We live in NC, the land of coastal and fescue, and refuse to feed either. All our hay is O/A or Timothy blends sourced from mid west or Canada.
Horses are blanketed or turn-out sheeted as required. Fly masks are put on/off as owner requests.
Amenities include access to 110x210 grass arena with jumps and LED lights. We have direct access to the amazing local trail network (over 4000 acres of horse-only property with lakes, fields, trails, jumps, etc). Open trainer policy. Tack room is over sized, dust free, and has a/c and heat. There’s a washer/dryer, and saddlepads are washed after every ride. Boarders have a bridle rack, saddle rack, and room for a large tack trunk. They can store additional tack/equipment (off season blankets, etc) in non-HVAC room if they need more room than tack room can accommodate.
We hold for routine vet/farrier appointments as long as boarder uses the barn’s vet and farrier.
We offer trailering services, at a reduced rate if boarder is attending same show/clinic I am.
If Boarders are traveling or don’t visit frequently, we send regular photos and updates.
DH was a (human) medical professional and is great at medical response to injuries/illness and all followup care.
For all the above we charge a little above the ‘average’ local costs. The direct access to the trail network does drive cost/demand up, as does the fact we don’t feed coastal hay. More show-oriented barns, places with covered/indoor, or required lesson/training packages tend to run about 25-50% more. Places that offer pasture board and/or crap hay tend to be about half the price. I’d rather have empty stalls than lose money (my time is not free) or have the wrong boarder (horse or owner) to deal with.