I'm not much of an expert on anything, but I have played around with SSI a bit. The problem may be that the server that is parsing the SSI commands has no idea where to look for the value of $id. What do you get back in the id variable to your cgi script?
Here is a helpful tutorial on SSI in case you need it. (You may be an expert for all I know)
The solution may lie in using JavaScript to fetch the data, or some other thinking-out-of-the-box approach.