This is more of an industrial mining question than an RPG one, but pretty much the gaming answer is: whatever you want it to be.
The thing with "gold ore" is that it's a mixture of valuable gold and worthless rock and that ratio decides what you can get for the ore.
Going by this site (which, of course, talks about modern day mining) a chunk of high grade gold ore would be worth as little as $0.75 per pound. At those prices, your whole cart is worth between a few coppers to a few silvers.
However, it's pretty likely that your game is set in a more medieval society, where the gold ore mined is much richer than that, due to less efficient refining techniques only making those worth it. Or, you just want to reward them for their hard work. So you could set it a bit higher; maybe a few gold pieces because of exceptional quality.
But really, gold ore isn't generally very valuable; you need to process a lot of rock to get anywhere. Then again; gold ore normally also just looks like normal rocks, which makes me wonder why your players were hauling it up in the first place... could it be you told them it was visibly sparkling with gold or something? That's certainly the way it's often depicted in games and movies, and would also explain why people think it's valuable. If it was carted up under those assumptions, definitely just make it a valuable thing.
I mean; people expect gold ore to be valuable, even if it isn't, and this is a game. Might as well make it valuable just because that's what everybody expects. That's the good thing about asking on the RPG exchange instead of Earth Sciences or Economics; we get to tell you the price that makes your game most fun. And that should probably match expectations much more than reality.