How much can I get from selling my handguns at a gun shop?

I am planning on selling my Taurus pt 740 so I can get a Glock 19. How much would I get from selling a Taurus pt 740 or any hand gun in general? Do they like take off 30% of the regular price? It was about 359+tax for the gun. How much should I offer to sell it for, and where is the lowest I should go for selling it at a local gun shop?