I have privately played a bit with cheap 433MHz hardware that I have bought at a nearby shop. The kind you may find in radio-controlled light dimmers, wireless door bells or similar. I think I payed maybe §25 to get a receiver and a transmitter but I have seen cheaper products too.

On the ones I got, it was quite easy to locate the data in/out signals, allowing me to use a generic processor on either side to transmit "any" data instead of the rather dumb protocol the equipment was built to use.

You can improve the reliability of the transmission quite a lot (and/or the working range) by sending multiple packets with the same information and either require the receiver to detect at least n identical packets, or add checksum or even error-correction in the packets.

For me, it works quite ok to control a bit of equipment at up to 100m free sight. The things I want to control is closer but not free sight, so I invested in two RF sets to be able to get back an acknowledge.

I have played a bit with other receivers/transmitters too with way better reception sensitivity (and better frequency bands) but that have been "left-overs" from a previos job and would be a bit expensive to buy.