How much it costs depends on why the house is too cold and what kind of forced air system it has.
If it is uninsulated,drafty and has no solar heat gain,it will be relatively expensive.
If it is airtight and well insulated but has no decent solar heat gain then it won’t be too bad.Really this is what you want unless perhaps you are right on the coast. In hot climates it is best to have poor solar heat gain and good insulation/high efficiency hvac.
Some houses could away with little heating and no AC in San Diego if everything comes together right for their climate zone.