It would probably be a significant increase in bandwidth to use RDP, as you are essentially downloading the page once and then streaming it out as a video, but it depends on the use case. On a local network that second step is probably easy to discount as unimportant.
If all the users are on the same network and sharing the same internet connection then using RDP probably still won't reduce bandwidth as each user should have their own profile (for security and to prevent abuse) and web browser with their own dedicated cache, which will mean that they all use the same external bandwidth. It also means that they are at the mercy of the performance of that one machine, turning a potentially smooth "local" experience into a slow, janky, juddery video.
If the users are "sharing" one RDP session then it could mean a reduction in bandwidth, as things are cached for that one profile, but you loose productivity if more than one person needs to use it at a time as they have to wait for others to finish.
What you actually want to do is proxy the connections and cache as much data as possible to each client through that proxy. "live" database connections are unlikely to get cached, but CMS "static" content probably will.
