The library at Fayetville University has a lot of archival material pertaining to orders regulating horses/mules, trade w/ civilians, soldiers, etc. There's calvary info there too. (Archives)

My great, great grandfather John Merrell lived in Texas co Mo. during the civil war & he sold stock to the Union army during the war. He also sold stock to the calvary leaving out of Independence during the Indian wars following the end of the war.

Two of my uncles, his sons, were pony express riders. One was killed by indians in Kansas.

I don't have any information pertaining to prices, etc. that would be much use to you. Different breeds were used for different things. The Scot's were used for pulling wagons, etc. because they were larger (like Clysdales)& the 1/4 & thoro breds were used for fast trips & were smaller.

All stock used by the calvary had to be inspected by the Adj. Gen in charge. If a soldier used his own stock, he was compensated for the use of it or if it was killed or injured in battle, etc.

Never really thought much about the stock used. I'd start w/ those archives at Fayetville that have a lot of records for the stock used. Ft. Smith should have some info too. The genealogical library has tons of info on micro-film & texts regarding the Fort activity including civil war infor. Good Luck!