Unfortunately calculating equivalency between two different movements or even the same movement by one person on different days is near impossible. You would also have to define exactly what you meant by equivalent. Amount of ATP burned, number of protein molecules generated, amount of work done, effort required, etc. I suppose the most common way to compare exercises is through muscle/strength gained by comparing different workout routines.
Effort?
Based on how long you have been doing an exercise, your body will have different neural adaptations, meaning certain exercises will be easier for you because your body is optimized for what you have already done. Combine this with the fact that everyone's bone structures and muscle/fat distributions are different. The same exercise on different people is incomparable, or even the same exercise that you performed two days ago is incomparable due to your most recent adaptations in terms of effort.
Work Done?
Hypothetically if you get a detailed body scan, you could do the math to see how much 'work' is required for a rep based on where your weight is distributed, the angle of your pushup, etc. The problem with this, is you run into a bit of an issue with terms from different worlds when you try to calculate work done in static holds. You could try going down the rabbit hole and ask something along the lines of Do static holds build power, or static work done during a Bulgarian split squat answered on both the Physics Stack Exchange and here, or Calculating amount of work done in gym workouts. Work in physics is Force * Distance. Static holds don't move, so technically there is no work being done according to physics.
Muscle Generated
This is likely the most common way of determining how effective certain exercises are compared to others. You can find study after study comparing how well different subjects responded to different types of workouts, movements, etc. In a review of current literature of isometric strength training, they mention that in order to increase hypertrophy, you should perform isometric training at 70-75% of of maximum voluntary contraction between 3-30 seconds per pre, at a total of 30-90s per session. This is not saying that different rep ranges don't improve neural adaptions or strength gains, it is saying that hypertrophy is optimized at that range.
