Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.
It can also be put on as insulation in attics and crawlspaces.
Combine that with a full ducting audit if any of your ducting pierces the envelope, a full intrusions audit for power boxes and the like, and new windows/house sheathing/ proper roofing, you can get very close to as good as a new built.
Most of these I would not suggest doing as a DIY, therefore I still say it is expensive, and even moreso if you have lathe and plaster instead of more modern drywall or encounter any of a myriad of issues likely to be uncovered when doing these things to a very old house.
