7rd with stepped grip or later 8rd with standard grip? To some it will make a difference.
Most likely if it never had a rail, its the earlier 7rd. But I have never seen Hogue grips on the 7rd variant - if the Hogue grips have been cut down, subtract as the correct stepped grips are hard to come by.
Most of the later 8rd 3913TSW had rail and LCI port - I have seen with rail absent and holes nicely plugged. I have also seen with holes poorly plugged (subtract or avoid). Hogue grips as well as correct SW grips are available for the 8rd variant.
I have seen them go $550-750 depending on condition, number of mags (2 is standard - add $50 for each additional correct SW mag or subtract if only 1 mag), box + paperwork.
Also depends on how purchasing - person to person cash with no tax will be best option and $650 probably good.
But retail add tax, and auction like GB, add shipping and credit card fee and tax and GB fee and receiving FFL fee = probably adds $125-150 so a $650 purchase may be more like $800+ which would be high for me.
Most LGS will bargain on used firearms - but that may depend on how long it has been there and condition - I would offer $600 and see if they bite, but if you want it pay the $650.