The only one that I'm aware of that will downmix the audio is Crestron's Digital Media switchers.
And how expensive: A fully populated 8x8 HDMI matrix switcher with downmix cards has a list price of $15,000. If you want to use their built-in HDMI over Cat5e (glorified and fancied HDBaseT) for inputs or outputs, then the price goes up. Of course, you can make it cheaper if you have less inputs or outputs. Or you can make it much more expensive going up to a larger switcher.
Plus, it is only controllable from a Crestron processor, so you'd need one of those and programming to make it work with something else.