The Reds are signing the Southampton defender for a club record £75m.
Wagner joked that his own centre-back Christopher Schindler was worth even more.
"I have to be careful now what I say because (Van Dijk) is Jurgen's player," Wagner said.
"I think everybody knows what a wonderful defender he is, but to be honest these are numbers I never deal with and I never thought about.
"This is why I can't really say much about it and judge it. This is a surprise and there is someone who pays it! They came together and now hopefully he can help Liverpool."
When asked how much he thought Schindler is worth, Wagner replied: "Seventy-six (million).
"He was, for me, the stand out centre-back in the last Championship season and what I've seen in this Premier League season is outstanding as well."