It's always hard to estimate without lots of detailed photos or having the gun in hand, but it looks like the breechblock has a flaw, it has widespread light pitting, and there is a crack in the wood (and cracks don't tend to get better). It also appears to have been cleaned, but the markings I can see still look OK.
I recommend you look at the "completed" listings on a site like Gunbroker to see what comparable guns actually sold for. You will probably see that prices for all the standard Spencer carbines (1860, 1865, 1867, 1868/NM) fall in the $1100-$2000 range unless their condition is either very very good or very very bad.
The one you are looking at strikes me as more of a shooter than a collector piece (assuming the bore and mechanics are good), and I would expect to find a similar carbine for somewhere around $1300-1500; I doubt you'll find one for under $1100 and I would consider $2000 pretty high.