I am assuming you mean a basement without permits or illegal basement suite?
There is no set figure... the quality of the basement development will make more of a difference. Either way, you will need to disclose whatever it is that makes it "illegal" and different buyers will weight the downsides with different value. If you had all professionals come in and simply didn't pull permits, it might not be that difficult to get permits after the fact. If it is an illegal basement suite, it really won't hurt the value as many basement suites are illegal. A legal basement suite will have added value however it is not common that the added value offsets the money spent to make it legal.
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