Kast is designed to be low impact. Being in a party consumes a variable amount of upload and download bandwidth depending on how many videos you are viewing simultaneously and if you are broadcasting video. Videos that are turned off do NOT consume bandwidth.
Each video stream consumes 1 Mbps. When a premium subscriber is streaming, the bandwidth consumed is increased to 2.5 Mbps of upload for the host and 2.5 Mbps for the viewers.
Audio streams are much smaller and occupy approximately 100 kb/s per stream both uploading and downloading.