I want to buy a WiFi USB adapter which would allow me to monitor the packets using tshark, and it should preferably be cheap. I'd also want to know if there are cheap WiFi USB adapter with antenna under 8-9 US$.
There's a list of wifi adapters that work with the pi here.
I'm sure you can manage to shop for them online yourself. Under $8-9 US is probably a little low, but there are many of them in the $10-15 range. I would not worry about an antenna, the pi USB ports are underpowered so you want a small adapter, unless you attach a powered hub.