The first year 1895s in 45-70 don't seem to command any additional premium. The serial number on the first years are preceded by the letter "B". Some say this indicates Ballard rifling, but I have not been able to verify this. Regardless, Ballard rifling should have only 6 grooves to the microgroove's 13.
Those who insist on Ballard rifling may be willing to pay a bit more for that rifling, but overall, the ones I've seen for sale have not gone for more than the later ones. Condition is of value, but only if you don't intend to use it. If she's to be a safe queen, buy one with the highest condition and best looking wood you can find. But that additional value, if any, will be gone after the first hunting trip, or the first two trips to the range.
I paid 600.00 for mine, 1975 DOM, about 5 years ago. The ones I've seen go for 600-700, mainly because Ohio decided to allow them for deer hunting a few years ago. Those who want to retire their slug guns are still buying them.
I did pass on a first year, B prefix, about 6 years ago for 300.00. Didn't know what it was at the time, but the muzzle looked like it had been used for digging potatoes--dings, dents, and completely in the white. Still I'd have bought it had I known it was a first year.
Everyone who buys firearms for investments takes a risk. The risk is that they will be not be worth any more than purchase price when you want to sell them. The few exceptions are collector weapons that are already worth a lot due to age, condition, factory letters, provenance, (owned by someone famous or used at an important historical event, say, Custer's personal Colt that he had at Little Big Horn, and it can be proven). A few, like model 52 Winchester Sporters have become very desirable, but the ultimate price may have peaked, along with WW1 and WW2 1911s made by certain factories.
It's far too early for Marlins made in the last 75 years to have appreciated much in value beyond their original sales price converted to today's dollars. There were just too many made and too many still around.
My personal opinion is that there will be a lot of 40's, 50's, and later firearms coming onto the market from the estate sales of their original owners. By and large their kids, and certainly their grandkids don't want them, as the Boomers die off. Revolvers are in this category right now.
A good rule is never to pay more than what an item is worth to you. What enjoyment it will bring, or what utility. Even now, many of my guns would not sell for their purchase prices. But the enjoyment and use of them has been more than worth the difference. Very few have appreciated in value, but not the ones I would have expected to.
If you like it, and have the 800.00, buy it. Just don't hang your hat on it being worth that or more when you choose to sell it.