IF you can get them down on the price, and IF you reload your own ammunition, I can put you on to a few recipes for that 45-70 barrel that won't give you a religious experience every time you pull the trigger.
A 300 grain cast bullet over a starting load of Trail Boss powder will not be at all bad.
If your only option is factory ammo, then I would walk away from that package, and keep looking for something else.
Counter offer them $350, and see if they bite. You can always sell the barrel here for $150 and use that money towards something else.
If you're going to get serious about this Contender bug, you will really want to start handloading.