Shrug, Smile, and say
"I'm happy to look over your NDA. This is what I do. Just like I would never expect you to provide me services for free, I expect to be compensated when my services are provided."
Then move on if they are still argumentative.
--- and that'll be $3 for looking over your question :) ----
This review is for your benefit, not theirs. You are not providing a service for them here, you are managing your own business. To some extent, they don't care how well you review it as long as you sign it. The exposure is yours so why should they pay for it.
Costs like this should be accounted for in your operating budget and feed into your rates, but ...