It really depends on the particular LNL or NLN combo, regardless of whether it is 2, 3, 4 or 5 characters ... for example, H5N1 in any of the better TLDs would be worth $xxx or more easily these days.
To answer that question you have to ask yourself:
What a LNL name like c5t or j8a can be used for ?
Most of the time: nothing.
Exceptions include terms like b2bor b2c. But most combinations are worthless IMHO. Again, 3-char names are rare, but just because they are rare do not make them valuable.