If you did a consensus amoungst plumbers when you hear of a water heater older than 5,6,7….10 years or older, your money is best invested in the replacement of the heater.
Many external items can leak on the piping leading to the connections of the water heater, parts can become defective. But when the tank itself starts to leak, that heater is not worth the time of even considering a repair.
It is a steel tank, the anode rod disappeared years ago. From that point on the degredation of that tank continues to the point of leaking with no ability to repair.
Can’t give you guidance on pricing as it’s too unpredictable from area to area what the going rates are.
Consider 20 years of use as genuine luck that it last that long. The inefficiency of that heater right now is like a 8 cyl cadillac running on 4. Costly at best and you’ll realize this with the new heater.