Most English/American last names are taken but you can find last names or surnames of other cultures. One can get the source data from here: https://forebears.io/earth/surnames
Last names can be really interesting because you are more likely to get a buyer for last names that first names.
- Will.eth can only be bought by people named Will
- Smith can be bought by all the Smiths' in the word which is a larger number of people
And with .eth domains, your can even do a sub domain making us possible to do
So I would say last names are at least 10 times more valuable then first names.
There are some good Last names available with short length. If you have the holding power, I would suggest to buy them quickly.