Hi Ronv, I'll throw out a reply as no one has yet. I was hoping someone with more knowledge could shed some light and hopefully someone will yet because I also seek more info in this area.
I acquired several hundred Concorde covers and have tried to figure out a value. Typically on eBay
they go from around $5-50. Things that increase value is signatures of the crew and this goes up if the cover was a return flight that was also signed by the return crew. Also watch for things like if you have a cover from the flight number that crashed, or if a famous celebrity or someone important was on the flight. That's more just the trends I've seen rather than any concrete value per cover / flight. Usually, as with all covers, any kind of uniqueness or oddity you can find will up the value. But the $5-20 range is the usual amount I see them go for, up to the $50 area depending on above factors.