I had a good relationship with my MIL. She once asked me to do the band uniforms for her last born (20 yrs younger than my hubby) and I did -- for nothing.
I would ask her to provide the material - she wanted specific kind, but if she couldn't afford that, then I would buy it for her. I never charged her for my work, but she would have paid me if I had.
If your time is especially limited - and she wants them right away, then I would charge her half of what you would charge anyone else.