Recently, there is a big demand on contactless devices for health safety, therefore developing a low-cost contactless breathing rate sensor will have a great benefit for many patients and healthcare workers. In this paper, we propose a contactless sub-6~GHz breathing rate sensor with an implementation using a low-cost USRP device B205-mini. A detailed performance analysis of the proposed system with different sensor algorithms and local oscillator frequencies is also presented. The proposed system estimates the channel phase shift at a randomly selected frequency and detects the presence of low frequency oscillations in the estimated phase shift. Compared to 24~GHz or 77~GHz FMCW radar based systems using distance measurements, the proposed system is simpler, can be built by using more economical RF components, and requires lower sampling frequencies. Another key advantage of the proposed system is that, even a very narrow unused frequency band is enough for the operation of the sensor. When operated at frequencies shared by other devices, the proposed system can turn off the transmitter at randomly selected intervals to detect the presence of other transmission activities, and then can switch to a different operating frequency. We provide both Python and Octave/Matlab based implementations which are available in a public GitHub repo. Finally, we demonstrate the effectiveness of the proposed system using real measurements.