When Wayne Brady offers LMAD contestants $500 to not take the deal. why don't contestants counter w/ $2000?

The premise of the show is that the buyouts offered are the result of mathematics.  They are not just some guy randomly picking a number.  It's based on which possible values are remaining in circulation.  So if they offer you $500, then the EXPECTED value of the remaining options is likely right around $500...maybe slightly less or more depending on their choices, but you can bet it's very close to $500.

So if you counter for $2000, they'd have no incentive to agree.  Further, it would ruin the format of the show, and they'd just cut that from the final show.

Because the contestants don't get to negotiate. They can accept or decline any offer.