Edit: the directivity of such a large telescope is also big. Around 45 dBi at the lowest frequency (70 MHz). That means the telescope is not very sensitive to radiation from sources near the horizon and more sensitive to radiation coming from it's pointing direction (closer to zenith).
For FAST, one must understand that this could not be smaller potatoes for the PRC, as big as it is. A hundred million dollars for a government project is a rounding error. However, the excellence of the design and it being the best in the world on some metrics has engaged the Chinese public in a huge way. Consider how popular space is with the public in the US and Europe, and that the PRC has not really had a world leading facility (Hubble, Keck/VLT, VLA, ALMA, etc.) to engage that latent interest. I have heard an anecdote from an astronomer that they have gotten so much bang for their bunk in this way, that the government may simply build a second one and keep the first for tourists!