I new take generally costs $20-$35, and that is generally empty.  If the tank is full and open, no gas should flow out.  There was a code passed years ago that said all tanks produced must have a "back flow preventer valve".  If there is gas in the tank, assuming you bought a standard 20 pound tank, it should weigh somewhere around 27-30 pounds.  If it has not been filled before, you should have the tank purged before filling it.  If you have a place that will fill it on the spot, that is usually cheaper than trading it for a full tank, plus you get to keep your new tank and not exchange it for an old grungy one.