Question: How much does it cost to respond to the PCT search report / written report?
Answer: There is no requirement to respond to a PCT search report / written report.
One option is to file amended claims in an Article 19 amendment. Assuming that you already have the amended claims drafted from a corresponding US application, the cost is minimal perhaps $250. The main benefit is to put an updated claim set on file, so that the you don’t need to make preliminary amendments when entering national phases in Europe, China, Japan, etc. Another option Is to file a Chapter II amendment, which triggers another search report/written report from the PCT. The cost of filing a Chapter II amendment is likely $1,040 out of pocket and about another $1,000 to prepare and file the response was again assuming that you already have the amended claims drafted from a corresponding application.
If the references cited by the PCT require additional amendments beyond those already made in the US prosecution, the cost could be more, depending on the closeness of the prior art, and the extent of the amendments. But we would have to make those changes in the US prosecution anyway, because of the duty to cite all known, non—cumulative prior art to the USPTO.
If you file neither Article 19 nor Chapter II, you would probably need to make any claim amendments at the national / regional phase, when filing in Europe, China, Japan, etc. And of course each of the foreign associates would probably charge a couple thousand dollars for the preliminary amendments.