Technology isn't an issue on this. Depending on what RFID technology is used, read range wouldn't be an issue. Since cattle life is relatively short, active tags can be used which also address some of the concerns of a single person taking the readings. Also readers can be attached to a trailer, punch a button and read all tags within range (which can be large or small).
Uploading data, you are right, it is very small. A few hundred bits per cow is all that would be needed. Even a old 300 baud modem could upload all the data relatively quickly.
As you also said cost is quite low compared to the operating costs of a ranch. RFID tags are pretty cheap now, $0.10 for passive tags and $10+ for active tags. Readers are $500 to $2k, but those are a one time purchase. If a rancher can't absorb the one time cost, they are really hurting, and the USDA could always subsidize the readers.