NOS A9's with the oriiginal needle bearing design can usually go for at least 90 or just under 100 bucks at eBay.
What is different here is, the Galli labeling on your headset can actually lower its value potential as Galli was pretty much considered a second tier components maker, compared to Campagnolo.... while Stronglight was considered 1st tier, when it comes to French components. Exactly the same component physically, but different value perception just because of the brand labeling.
As an NOS Galli headset, it might bring in about $75 bucks max (and it will take a longer time to sell), but as a Stronglight headset, up to 100 bucks.....
A used Galli version in very good condition will most likely sell for around 40 bucks, while the Stronglight might still be able to pull in 60+.
French threaded/sized version usually go for less because of less general demand for them....