Starting the suggestions.

£120-140?

I have paid a bit less than that for non-mint but nice 6s. So that may be on the low side. But.

There are a lot around.

They are not that collectable.

And much more than that you are getting into FWB65 or Diana 10 territory - a lot of us like older 10M pistols as shooters more than collectors items. And you don't have the box etc that motivates a lot of people to spend money.