Let's use your numbers. Say that two years ago computing costs were $50M, and encoding was $46M of that. Now say that their costs are currently $100M, but the encoding workload grew 6X. Under the old system, that would have cost $276M, but under the new system it is on $22M. That would be a 92% savings, and would totally be in line given that in the last few years they have drastically increased their machine learning output, which would have overtaken encoding work.
If the EC2 hosts have already been reserved then Netflix "cloud costs" would be relatively stable. What you might see is a reduced rate of increase y/y.
2. Keep in mind Amdhal's Law (or rather a slight variant). The absolute reduction has to be weighted against the percentage of resource usage that has been reduced.
(All numbers are made up) If previously Netflix was paying 3MM for encoding, and now they are paying 0MM; Compared to an annual 30MM on streaming with a ~25% y/y growth, you wouldn't notice the missing 3MM unless it was pointed out.
Servers barely even register as far as costs go, and also, it's a 92% savings over what the cost would have been without the system, not an absolute 92%. The farm keeps growing all the time as movies become higher resolution and more encodings are supported.