Effective antennas are reasonably easy to build from scratch (depending on the frequency band), and are commercially available in an insane number of variations. Either way is very cheap - as little as a few dollars.
Software defined radios can be had for as little as $10-20 for simple receive only types, up to several hundred for substantially nicer transceivers. They can also be $1000+ for special applications or R&D.
Software defined radios can be had for as little as $10-20 for simple receive only types, up to several hundred for substantially nicer transceivers. They can also be $1000+ for special applications or R&D.