Are slugs or sabot slugs made to come out of rifled barrels or smooth bore? Which do you find are more accurate. I know, different guns pattern differently.
Foster style slugs(rifled slugs) are made to be shot form a smoot bore barrel. Sabot slugs are made to be shot from a rifled barrel and are alot more accurate then foster style slugs.

Is buckshot made to come through smooth bore or rifled barrels?
Buckshot is for a smooth bore barrel.

What is a price range I should be ready to pay for a rifled barrel?
If you look around you can get a good sued one for around $100-$125. For a new one i wouldnt go over $200.