I got one cheap RFID reader from the bay (probably some old automotive surplus). This RFID reader is exactly the one discussed in this post on the Arduino Forum. Not too much info on it: some description and photo with possible modding.
It seems very interesting. Here is a picture mapping the internal architecture:
After configuring it, up to now I am able to let it recognize valid fobs and light the LED. Now I'd like to realize a security lock (pilot an electric bolt). I'd also like to connect it to Arduino to extract key fob info from it (as discussed in the forum, it generates the recognized unique fob code and also an internal-generated unique code).
UPDATED: it's not only on the bay. I eventually ended up discovering it's made in Germany (here) and it's very cheap.