Ok first of all.
Not sure where you are, but in my area your gas bill comes with a fixed charge of $30, usage is on top of that. So if you have ZERO usage in a month you still get to pay $30 a month. So that being said going from a gas bill that is 80 to 50 is acutally a huge reduction. The usage would be 50 down to 20, so a 60% reduction.
1. Buy a temp gun, they cost like $10 bucks on amazon and are a generally useful tool. They come with a laser pointer attachment so you can get exact readings. If you want to get really fancy have an enegry audit done. My utility offers them for $50 bucks and I think they give you a bunch of free stuff (LEDs thermostat etc) so the cost is about zero.
2. Heat loss is largest in ceiling, then walls, then windows. So target those things first. Blown in insulation is about the cheapest and easiest thing you can do. I MIGHT take you a day to do and im including time to pick up and return the blower. Crawlspaces are not fun to insulate, but it can be done. I would get rolled insulation or rigid foam. Prepare to get dirty and spend a bunch of time in awkward positions! Weather stripping around doors and a caulk gun is also very cost efficient.
Windows are nice to replace, but not a big winner in the energy savings department. What IS nice about new windows is the fact they all have screens, open, shut, lock, etc easily and reduce outside noise. So they are nice to have but dont expect huge enery savings. I have done windows twice, I did not regret doing them, but the energy savings were simply not there.
You can also blow insulation into wall cavities but you have to have a 3 inch hole every 16 inches, and that makes for a lot of patching and painting. You can also get a professional to do it, but that's gonna be 5k in labor and 1k in materials.
YMMV, good luck out there!