Water proofing a basement- is it a deal breaker?

[Sosina M.]

I am in the process of buying either a duplex or a single family house with a finished basement. I found this beautiful, fairly recently renovated single family house with a "finished basement" . But on inspection, we discovered that the basement was never water-proofed and there are some visible signs of water seepage in the unfinished parts of the basement. is this a deal breaker? how much of the cost is a fair amount to ask the seller to cover?