All depends of the context… If you licence a track where the producer just wishes to adjust the mix to the video, then imho, stems should not come with an additional “compositor” fee (i.e. come with full track on request) + eventually a small fee for the work you provide to extract the stems (hourly rate). But the video producer must understand that the stems should be used in context of your track: he cannot build a track on top of your stems or sample them for other projects. If he does and you allow it, then it should be one full track price per stem he uses that way.
Additionally, be careful with stems if you use RF sample libraries: all those I know do not allow the sounds to appear alone in a track that you would license, situation that can easily occur in some stems. Consult their EULA carefully or contact them if you have doubts…