If someone asks, I usually just name one or two items each for low, mid, and high price ranges and leave it up to the person to decide.
This. If you do it this way, you give options and they can choose one or a couple. I know even if my grandma was on a low budget, she would still want to get something bigger than a book... but she may not be able to afford a swing. Saying something like "well.. we need everything from books, to pacifiers, to a swing, to a bouncer, and even a crib. Whatever you are comfortable with would be very much appreciated!" Maybe not that exact wording, but I think that would be appropriate if you don't feel comfortable referring someone to the registry