Something to consider is the issue of condition and availability. Look what has happened to the values of M1903 rifles in the US. Many of these rifles were sported, bubba'd, plain used up. Original pre-1918 Springfields in fine condition are rare rifles, notwithstanding over a million having been made. They command high prices.
Of the hundreds of thousands of Mk. III Ross service rifles, how many have survived, unaltered, in untouched very good or better condition? Length and weight resulted in most in private hands being cut down. Everyone knew they were dangerous, and like Rodney Dangerfield and the Carcano, they got no respect.
Most intact Mk. III rifles that I have seen have been rode hard and put away wet. Many are pretty rough. I don't think that these are worth anything near $1500. As condition improves, I would expect the price to climb in proportion.
The last batch of Rosses to be imported were the Indian drill rifles; before that was the batch from Chile, ex HMS Canada. I suspect that the Latvian IIIBs are long gone. As far as the ones that went to Russia in the early 20s, apart from the few that made it back as surplus from the Spanish Civil War, they're likey gone forever. Apparently Mk. III rifles were sent to the Soviets during WW2. There MIGHT be some of these still in storage. Some Rosses were set up as target rifles by the Soviets during the 50s; one of these would be an interesting collectible and shooter. The Mk. III rifles sent to Britain for the Home Guard during WW2 seem to have been surplused, and many commercially sported in the 50s.
As far as condition goes, in my experience II** commercial target rifles, Mk. III Canadian Home Guard rifles, and original sporting rifles tend to be in the best condition. They were just not used as hard as service rifles.
