Well, that is open to debate and there is no standard that reductions are made for them. You are on your own there. Since you are avoiding paying a brokerage fee, how you come to putting a value on needed repairs is also your problem, and I can't help you with that. Somehow, some way, you and the seller will have to agree on the cost of needed repairs. Basically, you'll just have to guess, because you're not going to be able to get repair people to give you estimates in most cases, unless it's something really big.
Many people expect the surveyor to estimate repair costs.
While some surveyors are adept at estimating, the surveyor's job is determining the condition of the boat, and he has his hands plenty full doing that. To ask him to estimate costs on top of this will only distract him from his work, and in any case is only likely to be a guess. To estimate repair costs usually requires investigating what is wrong, taking things apart, and determining what it takes to fix. The surveyor doesn't know this any more than you do. The best procedure is to get everything together and make one, final adjustment.