i think this depends on the depth of your book or portfolio of past works.

1. if it is skinny , not good, or none existance, then you had better get used to doing this mock ups for free.

2. if you have a good portfolio or good book, then you should refuse point blank to work for free. If you are in the second category i get the impression that your sales effort has failed.

when starting out, it is pretty usual to do the first. later are you grow you shift over to the second category. if you do not then your professional career is having difficulties.