Like jguns2002, the real question here is if the $24k was for the Titan only or the negotiated price after accepting your trade-in. If the Frontier was an '06 and you only paid on it for a year, you were likely upside down unless you put some serious cash down when you originally purchased it. That in mind, here's how I see it....
$24k for Titan only = not so great deal
$24k with Frontier trade included = good deal
But at the end of the day, it really does come down to how you feel about the truck you now have. If you are happy with the truck and the deal, don't let someone here discourage you. It seems someone always gets a lower price and better deal somehow. And as a general rule of thumb for the future, don't get in the habit of trading vehicles every 1-2 years...it'll eat your lunch eventually and you'll end up as one of those poor lads that is $8k-10k upside down.