My mom has 'essential tremor' and it does make her hand shake, but I am not sure using a key is a big problem compared to other problems she has. She didn't get essential tremor until she was at least 75 years old, by which point her vision was very problematic. Any living-assist product she has purchased in the past ten years has been with her vision forefront on the list of problems to solve. (For example, she has attended a low-vision support group, she bought a clock with huge numbers on it, she got some yellow-tinted glasses that slide over her regular glasses, and she bought a purse that has a light inside.) If she could not see which key is which, and your product made a key more visible, and if she found that important, she might be in the market for something having to do with a key.
However, my guess is that it will be a long time before difficulty with a key per se would cause her to wish for a different key. It is not like you only have one chance to hit a key exactly into the keyhole or you don't get another chance. A shaking hand will make the key scratch back and forth a bit on the lockplate before it goes in, but that's about it. (I am thinking generally of a door key here. But the same point maintains if you are talking about a car key. It's not like you have to get the key in "or else." You can move the key back and forth a bit until it goes in without there being a penalty or difficulty.)
Possibly if there was a door lock that would allow someone to easily guide a key into it without needing precision in either seeing the lock well or fitting the key into the keyhole, that would be something she might consider buying and installing. I guess I am thinking of the lockplate being a funnel shape instead of a flat round shape. But this is not about the key, obviously, it is about the lock.
The only other key-related issue that she might possibly have is the ability to distinguish one key easily from another, and that might be helped by a top that slips over the key's top and is bigger or softer or in some other way distinct and possibly gives more to grasp. But iff too bulky, it might be a problem on her key ring. (However, I don't think my mom carries many keys. She does not drive, and perhaps her house keys are her only keys..)
A shaking hand does not necessarily mean dropping things. But if you think the person might have such a poor grip that dropping is a problem, perhaps your product could have a safety chain, something longer and finer than the key ring itself, to keep the key associated with the ring if the person loses his or her grip on the key.
As far as how much a device might cost before it didn't seem worth the money? If it's just something attached to the key only to improve the grip, maybe $5 to $10 (U.S.). If it's something that attaches the key to the key ring against the risk of dropping and also improves the grip, maybe $10 - $15. If it also has a beep that it makes when trying to find one's key ring, along with the other features, maybe $19-$24. (Forgetfulness of where one's keys are doesn't just just affect the elderly, and is a massively more common problem than just hand tremors making key use difficult.) If the product were a funnel-shaped lock that makes it simple to slide that key right in, and a locksmith can just take out the standard lock and put this in its place (so is a universal size) or it can just be attached to any lock, maybe $25-40 depending on what it was (if the whole lock mechanism or just an attachment.)