The cable was $200
here , and it acts as a dongle to unlock the software. The hardware itself is probably worth $50 - 100. If you Google "OBDII" you should be able to find other sources.
The hard part is finding software that can look at Mercedes-specific parts of the computer. Also, I believe (someone correct me if I'm wrong) that there are four different protocols that different auto manufacturers use to send data back and forth from the laptop. The generic scan tools can communicate with all four, but can only read the federally mandated OBDII codes. My VW specific program may not even read the basic codes, and definitely won't communicate with the transmission, radio, foglights, etc.
I hate to say it but if I keep the car long enough, I'll probably take the plunge someday. I haven't had any luck finding it on e-bay either.