Sure they are expensive. That's why the coal plants are willing to bid a negative price for their power, to have someone else deal with it. Rather than paying to build another tower.
The negative bids aren't coming from suppliers who can easily turn off, they have already turned off at this point.
And, cooling towers don't work by pumping hot water back into a lake. No need for a tower for that. They work by evaporating it, which lets you dump both the heat up to 100C, plus the latent heat of liquid -> gas.
