I bought my house, built in 1962, in 1999. The attic was first insulated with about 6 more inches of blown cellulose insulation. 4" Roll fiberglass was added over the top. The garage exterior were insulated with 4" fiberglass and OSB placed over the exposed insulation. The garage isn't heated, but does share a wall with the heated area, it was like adding another layer of insulation to an exterior wall. The 1962 furnace was replace with a 90% plus energy furnace. The water heater was moved to right below the shower and I got one of the ones with the blower vent. Now my shower water is hot about 2 seconds after I turn it on and there is no 4" open pipe going out the roof. Lastly new thermal efficient windows replaced the 40 year old ones.
Total cost was about 12k, I estimate savings to be about 2k last winter. One thing I notice now is that the snow remains on my roof longer than almost all in town (I live in Wisconsin). I also notice that rental houses lose their snow the fastest. It makes sense for an owner to insulate the house if they pay the bills, but for absentee landlords they can just rent cheap and let the students figure it out when they get $500 gas bill. As I learn more about building codes and rental licenses this is something that I realize can be changes at a local level. Our county gave out many thousands in emergency energy assistance last winter, maybe it is time to take a longer view and spend money to prevent the need.