The 6T70 / 6T75 TEHCM is a single unit that contains the transmission control module (TCM), shift solenoids, and pressure control switches. TEHCM stands for Transmission Electro-Hydraulic Control Module.
Due to the design of the 6T70 / 6T75 TEHCM, it's not possible to simply replace a failed component like a 6T70 / 6T75 pressure switch or solenoid. Instead, you have to install a new 6T70 / 6T75 TEHCM to make sure that all of the components work together properly and wear at the same rate. There are of course, many aftermarket kits that say they can allow you to replace individual components on the TEHCM. But that will only offer a short-term fix, because other components within the TEHCM may be damaged or worn as well. If you replace say, a pressure switch. It will start operating in a much higher capacity than the other switches on the TEHCM, which can lead to increased wear and component failure.
The 6T70 / 6T75 TEHCM is located inside of the transmission on the 6T70 / 6T75 valve body. Objectively, it is a fairly simple repair: simply remove the transmission pan, remove the valve body, unbolt the TEHCM from the valve body and install the new one. But it does take a bit of experience to not damage anything on the valve body or inside the transmission during the install. You must also have the correct tools and a clean workspace to do the job. If you try to go to the cheap route and just replace an individual solenoid or pressure switch, you'll have to pay the labor charge all over again when one of the other components fail.
Given there is not another estimate for the same work, you cannot determine if the quoted cost is too expensive. Always obtain 2-3 written estimates for the same work from other reputable transmission repair shops.