The way we [read I, since you're talking to 100% of the library
staff] "do it" in terms of charging for services is that many
[most] people make some decision short of paying for me to do the
research.  The descriptions of photos and manuscript records available
via a quick printout on our Argus system are often sufficient for
beginning-level research on any topic.  If it looks like we have a
great deal of material, the researcher usually decides to visit in person [and
schedule an appointment].  Often, the photo descriptions are sufficient
that the questioner can decide to order directly from the printout -
in which s/he sends a check & specifies the negative numbers, & our
Photo-Technician prints & ships the pix.

The key factor in our ability to turn around requests is the level of
automation of our collection records.  It would take me the 20-min.
maximum just to put together all the index cards for copying, if I
didn't have the database to rely on.  The ability to scan quickly the
database & give an educated estimate of the value of paying for a
custom search makes the decision more "rational" [in an economic sense]
for the customer.

Perhaps another factor is that we no longer equate local history with
genealogy in any of our museum collections here [though we once did].
There is a separate genealogical society in town, with fee-based
researchers and/or a public-access file of pedigree charts.  I'm not
going to wade into a crowd of angry Dixons or Devores to stake my
research reputation on whether Aunt Minnie was the second wife of
Uncle Josh or the third cousin of Sister Harriet.  I
can tell you every manuscript collection, photo, or artifact that might
mention or refer to all of the individuals, and I'll even add an
"Attribution" field tag to the photo of Aunt Minnie that somebody
says is really Sister Harriet [and put BOTH names in the subject field],
but I won't make the final call on ancestry.

A lot of people decide it's cheaper to pay the admission fee and
schedule a visit to our Museum, rather than ordering the research
done on my time.

--
Fred R. Reenstjerna, Research Librarian - Douglas County Museum
Roseburg, Oregon  USA - [opinions expressed are my own, and I
stand behind every one of them]