Reinsulating an older house

Hi! We think we need to replace or increase the amount of insulation in the home we are purchasing. I've been searching for an average cost of this type of upgrade in order to put a budget together and haven't been able to find anything.

We would like to avoid taking either the interior drywall or exterior siding off if possible but I'm not sure what our options are if we don't do that.

The house is a 2 level split with about 1900 total sq ft with access to the attic and there's a drop ceiling in the basement.

If more information is needed to provide a price range I'd be happy to provide it.

Thanks so much for the help!

Cassy