I know this is an old thread but I came across it just now as I was trying to get RS232 up and running on my Netduino.
If you're looking for a really cheap board to do the conversion from TTL (3V) to RS232 you could try this one from Futurlec which is only $4.90 and is working fine with my Netduino.
- Adam
can you post a picture of how to wire the TTL (3V) to RS232 board to the netduino?