That is an interesting question and one would have to look in a Blue Book for an idea, or search Gunbroker.
You have to get an account at Gunbroker to determine how much one actually sold for. I just looked and there are lots of M1893 Marlins asking $1200 or so and no bids.
All the time people confuse asking prices for value, when the value is what someone
"will pay for the thing", not what someone "
wants" for the thing.
I got into this at an estate sale, the people looked at Gunbroker but did not have an account. They made their price estimates based on the highest asking prices. My bud and I gave them offers based on our evaluations, the differences were hundreds of dollars off (about 70% of the asking prices) and there was unfortunately "no sale". When I got home I got into my Gunbroker account and looked at what these firearms actually sold for, and my bud and myself were "right reasonable" as you might say, nothing was selling at the hugely inflated asking prices. I will bet the sellers finally took the firearms to a local pawn shop and got half of what we offered, if they sold them at all.
If his rifle has 99% original blue and 99% period case colors, then the Marlins will have much more significant value. Looking again at Gunbroker, there are a lot of roaches for sale, only one had any of the case left. Lot of those asking $500 and no bids.
I saw some M36 and M1893's at the JM Davis Museum OK. My recollection was they were like new and the case colors were to kill for!
http://www.thegunmuseum.com/jmdavis2010/currentevents.html