I could get that down to 50-100 kB/s, that seems pretty reasonable.
For a 30 minute session, that's ~180 megabytes. So 6 hours of gameplay will break 1GB/month limit that many devices come with. Some low-end plans only come with 100MB of data or so. These numbers start approaching video on demand.
At $0.1 per MB over that, it would be $36 per hour of gameplay. That is, simply put, insane. Unless you're running a 1-900- business. Honest advice, if planning on distributing such an app, it better come with a huge warning, or you may bankrupt people.
For a RTS, with dedicated protocol, think tens of bytes per second instead.
Even if costs weren't a problem, network reliability is. A small delay will mean corresponding pile-up of data, which isn't biggest problem, but considering the way naive serialization works it may cause bursts of multiple megabytes on spikes.