100-450 or so
The contender barrels sell in a huge range depending on the condition, caliber and construction of the barrel and type of frame, Plus what extras are with it. In this case a 16" barrel alone can sell from $150 for a blued .45 colt to $350 + for a stainless 45-70. Add forend or scope or target sights and you can be in the $500 range. Contender frames sell for $250 - 325 for the 1st generation blued to near $500 for G2 frame in stainless. My suggestion would be to look on gunbroker.com, there is a dedicated contender category and you can get an accurate value with a bit of research.
$650, plus scope.
300 or so
300-400 USD
If you ever have the privilege of using a Leupold scope, you will know what a really good scope is. Leopold's are good. But not what they asked! BSA scopes are of GOOD quality. I have had BSA scopes on many of my guns, including a Thompson Center Contender in 45-70 Gov't. A very strong recoil load, and the BSA Gold I have has done great! Never missed a beat, and with a TC you only get 1 shot...it is a single shot pistol/rifle.
There are a couple of methods, First and easiest is a 1 piece base made for the TC Hawken. and screws to the existing rear sight holes Available from many of the large retailers like Midway USA Cabella's and Thompson Center. With the base you can choose your rings or even use a scope you may already have, and be able to remove it for cleaning the gun. The second is having the barrel taped for rings, several style are made for this application also, and from the same retailers.
buy a scope and scope mount, get an outer barrel sillencer extention, upgrade to a 6.03 milimeter tightbore barrel, and install a bipod.
There are several different models of the Mark X, however, most have a RECEIVER that is drilled and tapped for a scope- not the barrel.
Just bout one at auction with 4x32 tasco scope for $110
Yes it does have a red dot in the middle of the scope.
Assuming you mean the Remington 870 shotgun, with a scope- is your barrel rifled? If so, sabot slugs are generally the most accurate. If smoothbore, then rifled slugs will be most accurate.
You need to add a scope-stop to the barrel. This will stop the creep backwards.