I almost never do that, for two reasons. First, I don’t like it when people do that to me, especially early on in the relationship. Second, as a fundraiser, you don’t know the donor’s situation.
Instead, I will explain the cost of the project I am pitching, and say “I hope that you will consider making a significant gift.” That’s a healthier way to essentially get to the same place. The real trick is to size the project based on what you think the individual can do.
Some research can give you a sense of the order of magnitude of an affordable gift. It’s not just about net worth, though. A donor’s philanthropic funds may already be committed to another cause or organization. (I have one friend who prefers to make pledges so he won’t be asked again for five years.)
They key thing is not to make a donor uncomfortable or, worse still, annoy them by asking for too much. It can work the other way, too, to your advantage. You may be pleasantly surprised by a larger than expected gift. I have been.