There is only one sub $50 kit that could be considered high quality and with a good reputation for on-air use.
This is the QCX from QRP Labs.
https://www.qrp-labs.com/qcx.htmlI've not built one but have heard good accounts from people who have. It has many features that make it good for on-air work.
Eg single signal receiver, 5 watts output, frequency agile transmitter etc.
Much cheaper and widely on eBay is the Pixie. They're fun to build and modify. Not so fun on air as contacts will be hard.
It's very low power (about 0.5w), stuck on a single frequency and has a very broad receiver that may be overloaded by broadcast stations.
More detail at:
http://home.alphalink.com.au/~parkerp/projects/projpixie.htmThere are some kits of intermediate price and complexity but again you want a good receiver, frequency agility and decent power output.