Aren't .tk domains free? If so, they are worth nothing, since they are merely a redirection domain and did not require any cost to purchase in the first place. People would much rather have a .com or other higher level domain.
I really see no value in a .tk domain name. You can get a free one but with a popup most people wouldn't pay a cent for a .tk name. They would likely just go register for a free one and wouldn't mind the popup. I would guess the value at $0.00