Several factors complicate the otherwise simple analysis of cost per stream. First, you should analyze using the output codec or codecs, current and future. Many systems output H.264 quite competently but choke considerably with the much more complex HEVC codec. If AV1 may be in your future plans, you should prioritize a transcoder that outputs AV1 and compare cost per stream against all alternatives.
The second requirement is to use consistent output parameters. Some vendors quote throughput at 30 fps, some at 60 fps. Obviously, you need to use the same value for all transcoding options. As a rough rule of thumb, if a vendor quotes 60 fps, you can double the throughput for 30 fps, so a system that can output 8 1080p60 streams and likely output 16 1080p30 streams. Obviously, you should verify this before buying.
If a vendor quotes in streams and you’re outputting encoding ladders, it’s more complicated. Encoding ladders involve scaling to lower resolutions for the lower-quality rungs. If the transcoder performs scaling on-board, throughput should be greater than systems that scale using the host CPU, and you can deploy a less capable (and less expensive) host system.
The last consideration involves the concept of “operating point,” or the encoding parameters that you would likely use for your production, and the throughput and quality at those parameters. To explain, most transcoders include encoding options that trade off quality vs throughput much like presets do for x264 and x265. Choosing the optimal setting for your transcoding hardware is often a balance of throughput and bandwidth costs. That is, if a particular setting saves 10% bandwidth, it might make economic sense to encode using that setting even if it drops throughput by 10% and raises your capital cost accordingly. So, you’d want to compute your throughput numbers and cost per stream at that operating point.
In addition, many transcoders produce lower throughput when operating in low latency mode. If you’re transcoding for low-latency productions, you should ascertain whether the quoted figures in the spec sheets are for normal or low latency.
For these reasons, completing a thorough comparison requires a two-step analysis. Use spec sheet numbers to identify transcoders that you’d like to consider and acquire them for further testing. Once you have them in your labs you can identify the operating point for all candidates, test at these settings, and compare them accordingly.