Depends of the shop and size of the tank... Probably $4-8 usually for
an AL80... Either you buy a compressor because you actually *need* it
or you buy it for the convenience... If you're diving some place where
there are no nearby fill stations and just taking a lot of tanks with
you is not an option, having an engine powered compressor might be an
option that you should consider... If you get into mixing your own
gases, it might also be an option to consider if the compressor can
pump something other than air...

Quite a few.  Shops charge different rates but $5-$7 is about the going
rate.

I sell a LOT of compressors, oil and filters so maybe I can shed some
light on this.

Owning a compressor is not something you do to save money unless you
dive a LOT!  Most people buy them for convenience or safety.

Convenience in that sometimes it is just a pain in the butt to drive a
bunch of miles to the dive shop for another fill to make a second dive.
 Or that you are going go diving on a holiday (when the shop is
closed)and forgot to fill the tanks.  Or that you are heading off on a
weekend dive trip to a remote destination with no shop around and have a
choice of taking your single tank and a compressor or eight tanks. I
think you get the idea.

Safety is another matter.  While most dive shops have excellent air,
there are a few others that pump junk...not poisonous ... just not too
great.  When you change your own compressor oil and filters, you know
the condition of your air.

As far as saving money, that is a complicated subject.  Taking the $3090
cost of a new compressor out of the loop, you still have to purchase
filters and oil. The following numbers are high and are based on filling
at least 100 fills per year but:
$60 - breathing air outlet filter good for about 40 fills ($1.50/fill)
$13 - high grade synthetic oil change good for about 60 fills (.21/fill)
$13 - inlet filter to be changed at about 100 fills (.13/fill)

Of course, if you pump less air per year, the costs go up since each of
the above items are changed at least every year. At about 1000 hours
(about 3600 fills) you can count on a full overhaul and that will run
about $1500 or .41/fill.

So ... not factoring in the initial cost of the compressor, you are at
about $2.25 per fill.  If you throw away the compressor at the 1000 hour
mark instead of overhauling it,your cost per fill is now closer to $3.

Oh man!  I forgot to factor in the cost of gas ans oil for the engine or
electricity for the motor!!  Might as well add in another few cents per
fill.

So I guess to answer your question, it will take at 50-100 fills per
year to really save any money over going to a shop.

Hope that helps!

Oh ... an let me know when you need a compressor, oil or filters and I
will get you the best deal I can find for you.