The lab charges about $250 (or more) for the abutment which goes between the
implant body and the crown.  The lab, also, charges about $300 more to make
the crown fit the abutment.  The added cost is rolled back into the cost.
If the lab were to bill the patient for the cost of fabricating the crown
(instead of billing the dentist), the implant crown would probably be
cheaper than the "conventional" crown.

    As Dr. C mentioned, it is largely lab fees.  The time expenditure for
me is about the same.  Furthermore, the restorative dentist must often
interact with the surgeon to make sure the fixture is placed where
needed.  I recently had to spend a couple of hours in the office of a
surgeon who had placed a fixture in a suboptimal position along with the
patient in order to manage the complications.
    I normally will pass along the increased lab fee.  The only think I
must say is that I discuss the situation with the laboratory after they
have received my pickup impression to determine whether custom abutments
are necessary.  The difference in fee is generally $200-$300.

Steve

Lab charges more because they make a soft tissue model and because it
gives them an excuse. There are also extra parts needed for the lab
such as lab analogs, impression copings, healing caps add on top of
this shipping charges sales tax for labour and parts (the implant
itself is exempt from sales tax at least since last 4 years, but all
other components are taxed). Finally there is the implant abutment the
piece that screws onto the implant and on which the crown is made.
Sometimes this implant abutment needs to be custom made by the lab.
Additionally some implant systems such as Straumann, have addition
single use plastic tryin components to select the appropriate implant
abutment. These companies also charge quite a bit for these parts.
Cementation x-ray of the abutment seated and one of the crown seated on
the abutment is taken and one prior to loading the implant to make sure
it intergrated. All of these services are built into that fee and
remember once one of these parts gets opened it can not be returned to
the manufacturer. Sometimes it is necessary to order extra variety of
parts for your tooth so that a selection is available during your visit
and what ever is not used unopened is returned to the company with
shipping and restocking charges. All of this is passed on to you.