I had two old fridges more than 15 years old in my house. The downstairs one had been repaired a few years ago for a broken heater while my upstairs one had a recent problem with a stuck defrost timer which I replaced with a part bought on Amazon. After the repair, I noticed the fridge seals were in rough shape so I started searching Craigslist for some used fridges. Modern fridges were supposedly more energy efficient so I was looking for something newer but used to save some money. I found one for $100 that had a cooling problem in the fridge section, but the seller indicated the freezer section was fine. There was probably ice blocking air flow from the freezer to the fridge compartment that a manual defrost fixed easily. A few weeks, later I found a second one (same model) for $200.
My local utility, BC Hydro, had recently switched to smart meters and offered a $75 rebate for decreasing energy usage by 10% over a year. They also had a fridge buy-back program which would start in the spring and would pick up and give you $30 for an old running fridge (max two claims lifetime on your account).
So the energy savings after replacement were about 50%. My electricity bill went from between $150-$200 every 2 months to around $100. The electricity savings alone paid back the cost of the fridges not even counting the $135 in rebates.
Not only are modern fridges more efficient, they run less since they use thermostats to cool only when needed. Old fridges use timers and are on a cycle of either being fully on or off in defrosting mode.
Here is my energy usage graph. In Aug I had new tenants. They said the fridge was cranked all the way up so they turned it down. End of December, both fridges were replaced.