The model 6450 is simply a 645, run a search on gunbroker for 645 and you'll get an idea of what they sell for. My guess, based on what I have observed is that used 645's in good to very good condition fetch about $450-$600 depending on what they come with and any signs of wear and tear. for example, the gun alone with no extra mags and some scratches and obvious barrel wear would most likely go for the $450 price, possibly a little less. with all of the "goodies" and in pristine, unfired condition $600-$650 wouldn't be unreasonable.

These guns sold in the 1986-88 timeframe and back then, new, they sold for around $650 OTD.