Hello Etien!
I have seen a lot of solution to communicate with RF. It is very difficult to choose one. But I think, that 30$ is not high price for a good working pair of device.
A month ago I have also considered a device to transmit data. But I decide to buy GSM terminal, because it is easy to handle and I can work for a very long distance. Prices are also not so horrible as a 2 years ago. I pay about 300$ for each GSM terminal, but it is working very good, indeed.
You ask about Manchester and FSK. Manchester and FSK are the kind of signal modulation. You can learn about from internet sites. Type MANCHESTER and ENCODING or MODULATION (f.e. MANCHESTER+ENCODING) and you get about 1700 results. There are sites, which are expaining all kinds of RF modulations, too. Good luck!
Jacek