I'm looking for the average price of a horse, in British Pounds, around the year 1750. I'm interested in draft horses (they pull carts), not riding horses, but the difference in price between those two wouldn't hurt. And I'm more interested in colonial America rather than Europe, although I have no idea if the prices will be very different.
Prices are occasionally mentioned in mid-18th century journals; for example, David Zeisberger's journals, available here: Diary of David Zeisberger : a Moravian missionary among the Indians of Ohio. In this one you will discover that a buck skin is worth a dollar, which is repeated in various frontier sources between 1750 and 1800 that I have read; this is probably the origin of the colloquial "a buck is a dollar"!
Though less interesting to read, a more direct source are the estate inventories that were required when probating an estate. In researching early Detroit under the Americans, 1780-1830, I've studied many of these in the original. Godfroy Corbus, along with many others beginning in 1797, is available (free) on FamilySearch. These are digital images, but the records have only a primitive index.
Godfroy Corbus was a farmer; he had moved to Detroit from Pennsylvania; probably of Scots-Irish descent. His widow is one of my aunts.
On this record, Wayne County Probate file #67, from 1807 one finds: One bay horse $30; one grey horse $45; one bay mare $45; one sorrel ditto $45; One bay mare $45; one colt $10.
One yoke of oxen $50; one yoke and ring $1; One young yoke $30; one red cow with white $12; etc. A side saddle, for his wife? is listed at $10. The entire estate is worth $732.85.
The usual pound used at Detroit was the New York pound, worth $2.50. Some records list values in shillings and pounds as well as dollars, right up to 1830.
A probate record from 1825, the estate of Jesse Hicks, Wayne County Probate file #299, we find:
2 horses and plough harness $100; 1 yoke of oxen with yoke $45; 4 cows $44; 1 yoke of steers three years old $20.
Note that a yoke of oxen can do the plowing, but a team of horses does more work in a day, but also requires more care and is more expensive to feed. The yoke of three year old steers is only worth $20 because they are not yet fully trained or fully grown: they'll be called oxen when they are four years old and fully trained as a team, left and right.
From these records you can see that the price of a horse is stable during this period, but responds to the local demand. You should be able to find similar records for your region of interest.
Another source is military supply records, which are numerous for the Revolutionary War period. Every penny counted, and on the frontier they had to buy cattle and horses on a regular basis. See Frontier Advance on the Upper Ohio, 1778-1779, which includes transcriptions of many original documents, with sources.
This list of medieval prices indicates the price of a draught horse in the 13th century to be between 10s and 20s,, while this estimate of global inflation shows prices in 1750 being 8 or 9 times what they were 500 years earlier.
I note that the steady inflation from the early modern era is due to the influx of American silver into the European economy by Spain.
Also, I suspect that draught horses would be less expensive in the English colonies than in London, for example, and for the same reason that cost of living today is higher in New York City than in Hoboken.
Putting this all together gives an average price of about 80s (or 4 pounds sterling) for a draught horse in 1750, This will go up or down depending on the age and condition of the individual horse of course - don't look a gift horse in the mouth as they say. Harness and tack would all be additional, just like GPS and A/C in modern cars.
"pounds sterling" means exactly that - translated, 20 silver shillings, which was a silver coin slightly larger than a modern U.S. or Canadian dime I believe . From Wikipedia, pertaining to the 1816 "new" standard reminting of the shilling: "New silver coinage was to be of .925 (sterling) standard, with silver coins to be minted at 66 shillings to the pound. Hence, newly minted shillings weighed 87.273 grains or 5.655 grams.". Prior to 1816 the mintings were irregular.