Yours doesn't really have any collector value but depending on where you live I would say it would fetch mid 300's to low 400's.
The value of a Six series gun really depends on where in the US you are located. In places that they may be more common/easily found the value of a used Six with no box or papers may only be mid 200's to mid 300's and for a stainless model mid to upper 300's. In places where they may be harder to find/purchase a blued model may go high 300's to mid 400's and stainless mid 400's to low 500's. That would be for the standard models that are in good condition. Models that are scarcer, like the 9mm, PD marked or military contract guns will be higher especially if they are in pristine condition.
A collector will pay premium money for a Six series gun that in NIB/LNIB with all the paperwork and the shipping box, or for a really scarce/rare model even if it isn't in as good of condition.
Here's the deal. Great duty gun 30+ years ago, but with fixed sights, not very popular now, 'ceptins for folks like me, that if I found another low-back in pristine condition, I'd buy it just to put it in the safe, forever. 8)
Still, for the right price, ($300-$350), it could be a great pickup gun or nightstand gun, because in spite of being low-tech by todays standards, they remain excellent shooters.