The Glock 23 is a great gun, but I wouldn't make the trade if I were you.
First off, I wouldn't want to part with that Mk III - ever. Second, the only way I'd ever trade two guns for one is if I hated both the guns I was getting rid of.
Try to trade the CZ and throw a bit of cash at the deal, instead. You shouldn't have to pay much at all... maybe $150-200 max, and I think that's a high guess (but still better than trading the CZ and the Ruger).
I just went to the local Cabela's today, and they're selling new Gen3 FDE Glocks for $549, and they have a huge selection of em. I saw the 19, 22, 23, 26, 27, and there was another I didn't see the model. Might have been a 30. Point is, they're out there. Glocks were never expensive to begin with, and you can still find em for great prices.
ETA:
In all seriousness, if you like the idea of trading the CZ and throwing some cash at the deal, it wouldn't hurt to go to a Cabela's near you, if you have one. You won't get the most money for your gun, but I know from experience that Cabela's will give you more money for a gun if you plan to buy a new gun there the same day. So while they might give you $250-300 cash for the CZ, you might be able to get $350 in-store-credit toward a NIB G23. Maybe even a bit more. It wouldn't hurt to look into it.