Time and materials and expertise! You've learned how to handle huge skinny TVs without folding them up like a taco, and you know how to be sure this TV is handled safely. You may need to bring an assistant, so that will cost, too.

Why is this a small project, $20K or so, but two TVs at about $10k will be added. That makes it a $40K plus unknown handling and installation costs of the added TVs type project. Your skill at handling very fragile TVs is worth a lot. Your skill at knowing when to rent a Genie lift, when to PURCHASE padding, frinstance moving blankets, to make sure you don't taco the displays... all that, and the cost of insuring just this one project... all those things are costs.

If the client balks at paying for what you KNOW you need to do to ensure the TVs get installed without damage, then respectfully decline.

What kind of video signal is going to be displayed on these? What kind of technology are they? Another odd thing is that if the intention is to mount one TV above the other, the combined image make a 1.4 ratio image with a line down the middle.

"One-off" is a GREAT name for this project. It has many risks. If the client will not recognize this and realize you'll be using all your skills to get it done right, and you need to be compensated for that, then yeah, I'd hesitate to do it, too.