Interesting gun you have there.
The 1842 Springfield was the first widely issued percussion gun produced for the military. It was also the last smoothbore musket made for the military.
There were approx 275,000 total made, 172,000 at Springfield Armory. The others were made at Harpers Ferry.
It was originally a .69 caliber smoothbore with just a front sight. From 1866 thru 1859 about 14,180 were modified to add rifling to the bore. Of these, about 10,000 were equipped with the long range rear sight your gun seems to have.
In 2007, Flayderman's Guide to Antique American Firearms lists the value of the rifled 1842 Springfield at $1,000 in "Good" condition and $3,250 in Fine condition.
I would consider your gun to be in the "good" range if not slightly better. With inflation, that would make the value of your gun about $1400 - $1800.
Of course, the true value is what you can actually sell it for so if a buyer can't be found that will pay that much, the gun would be worth less.
I forgot to mention, the 1842 Springfield was also widely used in the Mexican/American war although the date on yours indicates it was made after that conflict was over. Your gun was very likely used in the War Between the States.
I'm moving this thread to the Firearms Identification section. That is where we talk about the values and history of real antique muzzleloaders.