NDAs are practically universal in the software consulting market. It would be borderline unprofessional† to refuse to sign one on principle. Two things you can do if you're really worried about this: (1) offer your own MNDA instead of their paper, or (2) pay $300 or so to get a lawyer to review their NDA.

If you go with option (2), you can buy yourself a little time to try to qualify the client prospect by saying you'll sign an NDA, but you'll have to have it reviewed at some expense before signing, so you'd like to make sure there's an actual deal there before signing.

† This is probably not exactly the word I want to use here.