Get an understanding in writing.  In California, few clients ever approach us who do not have some access to email.  Providing them with a written offer becomes quite easy in this day and age.

Give them a minimum cost and tell them what it covers.  Then tell them the hourly costs if efforts extend beyond that.  We too like to offer a standard 7-day turn-around from the day of order, but it is not always possible and we let the client know how work is proceeding during that period.

Offer to have them cap the research at a set time or fee, whichever is easier for them to deal with.

Open, clear communication is critical to a successful transaction and a happy client.