The goal was to get the wife without spending thousands of dollars on her, and Candie wasn't going to entertain offers for significantly less; he had no reason to deal with them over a paltry sum for a slave that, while basically worthless in Candie's eyes, was also a hassle to replace.
I heard a scenario on sports radio the other day; it was in reference to coaches in positions they were happy with getting to drive up their price to suitors, but it's applicable. If I'm selling my house, there's a certain value that I would expect to get for it. But if I'm just living in my house, the value of it doesn't concern me. If you want to buy my house and I don't want to sell, I'll quote you an astronomical value for it, because while it might not be worth that much on the market, it's also going to take significantly more than market value to get me out of a comfortable situation.
Applying that logic to the Candie scenario, he doesn't care about a few hundred, or even a couple thousand dollars, for his property. He's comfortable where he is, and Django and Schultz have nothing to interest him. Nothing, unless they pique his curiosity with an outrageous sum of money (bearing in mind they don't actually have any intention of paying thousands of dollars for any slave).
Granted, this ignores the possibility of just paying an outrageous sum of money for Broomhilda from the get-go, which I think Django would have been fine with given that it's his wife, and Candie probably would have been fine with too given his problems with Broomhilda. Clearly, they weren't hurting for cash; not only did Schultz have $12,000 in his wallet, after the guy took the money, there was still a shitload left over. I guess they figured Candie's pride and lust for power wouldn't let him reunite a black family if he knew the truth about who they were, regardless of money changing hands. It certainly seemed unnecessarily risky.