If I had one to sell, I'd think $100 would be a fair price. High enough to account for the fact that it's a rare, much sought after item - but no so expensive to try to make Ferrari money on an MG part. As rare as they are, I'd be more concerned about getting it to someone who would really appreciate it....someone trying to keep their car very original.
I've given away way more stuff than I've sold...but none of that stuff was rare