Ask HN: Charging clients for estimates?

Can anyone point me to resources, or offer suggestions here, on how best to go about doing this? I'm part of a (small) design/development shop - most of our clients are small to medium local businesses. I've read some stuff suggesting that it's best to charge for an estimate, the justification being that you need to expend significant energy to come up with something that accurately scopes the project, and that's of value to the client.