Is my 1989 Amana fridge actually worth keeping running?

You can figure out the cost without buying anything, and it takes about ten minutes of math. Find the nameplate tag inside or on the back and look for the amp rating. Multiply that by 120 volts and that's roughly what it draws when the compressor is running. Then do the dollar-bill test on the doors to gauge how well the gaskets seal, and just watch the thing over a cold beer one evening. Count how many minutes per hour the compressor actually runs. Old fridges like yours don't run constantly, they cycle. Take your running watts, multiply by the fraction of each hour it runs, multiply by 24, and that's your daily kWh. Times your utility's rate per kWh, times 30 days, and you've got your monthly number. It'll probaly be less scary than you're expecting, maybe somewhere in the same ballpark as a couple light bulbs left burning around the clock.

One thing to keep in mind with that amp rating on the tag, since it tripped me up when I did the same math on my mother-in-law's old GE, is that the nameplate figure is usually the maximum load, which can include the defrost heater and the lights, so it will overstate what the compressor actually pulls while it is running. Your real number will come out a bit friendlier than the raw calculation suggests. On the gasket, I woud not patch it, I would look up the model number and see if a replacement is still listed, because gaskets for those older Amanas were sold for years after the units were discontinued and swapping one is honestly a Saturday afternoon job with a hair dryer to help the new one relax into shape.

Patched gaskets on old units plenty of times over my 20 years doing service calls, and a hardware store fix is fine as a stopgap but never holds long, the tear always walks a little bigger. Check the model number on that tag inside, alot of those late 80s Amanas still have gaskets available from supply houses, and swapping one is honestly a 20 minute job with a hair dryer to soften the new one into shape. As for the unit dying on you, the ones I pulled out of kitchens that were still running ussually quit for one reason, a dead compressor start relay, which is a cheap part. The compressors themselves outlast everything else on the box.

Update: did the nameplate math like the first reply suggested, amps times 120, then factored in the point about it being a max load since the defrost heater inflates that number. Turns out it's not nearly as bad as I feared. Also ran the dollar-bill test and the tear was definatly leaking, so I looked up the model number and a replacement gasket exists. Ordered one instead of patching.