You do not specify what you intend to transmit there and back. If it is a voice, buy low-cost walkie-talkies operating at 430 MHz. Then you can increase the range by adding directional antennas.
If you need to transmit video/audio, again you can use low-cost "wireless cameras" and add better antennas.
If you need to transmit data (at what speed), you can use the above or first study a good book on communications design, to develop a good system.
Look for a radio-amateur club in your location. Radio hams know best how to make a low-cost working system you want.