Well, the first thought I had in my mind is that my old DEFA car alarm control worked several ten meters, with lousy antennas in both ends.
With a proper antenna you might get longer range even with small transmit effect. On 433 MHz the antenna dimensions are not any more huge, and getting a little extra gain should be relatively easy. There have been some antenna books in e-book collections recently.
Another thing is the sensitivity and, not least, the selectivity of your receiver. I noticed that the same car alarm controller did not perform well at all in some places with strong disturbing signals. (One horrible place was Toulon in France, where I had hard times to inactivate the alam even when pushing the remote on my windscreen. I wonder what kind of transmissions they have there - had no scanner with me on that trip.)
So, see both of these aspects and you might manage >100m with smaller power than 100mW. It saves batteries, too.
Ted