Yep did that one with a few sheets of Styrofoam paneling. Built a box that drops right over the stair opening. I probably have less than $25 invested with the Styrofoam, glue and 1/2 inch door sealing strips. In most cases new appliances will not save you much ROI over the old ones except when it comes to water. Newer toilets, shower heads and washing machines will save you a fortune. Over the years at work just changing out two toilets saved over $80 a month. At work I had a 10 year old top loader washer and I was using around 14K gallons of water. A month after getting a HE front loader my water usage dropped to about 6,500 gallons a month. Incandescent to CFL big drop, CFL to LED smaller drop. Going from the large four foot T12 florescent lights to T8's big drop. Four foot T8 to LED smaller but significant power savings.
Around me a number of people went with Solar on their roofs to save money on their electric bills. Had they spent at most $5,000 getting someone in, to go through their home with air sealing and insulation they would have been farther ahead in the long run. I needed a new AC unit a few years ago. It was a 10 SEER and I was asking about replacing it with a 16 SEER unit. The HVAC guys looked at me and said no get a 13 SEER unit. We use AC maybe three months a year, you'll never get your money back that you would spend for the higher efficency unit. Heard the same argument from them on my 80% furnace. Don't replace it until you need to because a 95% efficient furnace might save you a $100 a year yet cost you close to 4K to replace what you already have.