It is the wrong forum, but I'd guess you could do a little better in trade on a GSG 5 for a used handgun. The Ruger P89 is a blocky hunk of 9mm. They function well, but value on the used gun market is probably only around $250-275. They weren't very expensive back in the day when they had their best sales. I've got zero experience with the Sigma line, so can't comment there.
I think you could sell your GSG outright and have enough cash for a used trade in glock, or other trade in guns. Look for a bigger gun store in your area that sells used police guns. I've had good luck with quite a few police trade ins - Glocks, Sigs, S&W are the most commonly available. A lot of time, the guns will show more holster wear than they do internally. Makes them cheap because visually they aren't appealing, but internally will likely be good to go. If you go this route, make sure you field strip the gun you are interested in to see the internals