Depends on the type of CF, bowl size etc. The ones PA biodiesel sells are pressure-driven "spinner" types and do not preform as well as the larger bowl types (Absolute Centrifuge, USFiltermax, SimpleCentrifuge, WVODesigns). The larger bowls can process much more oil before cleaning is required. The bowl types clean oil much better, but are also quite a bit more expensive.

Mikeoil:
In 2005, I made my open bowl CF system with the SimpleCentrifuge bowl ($550), the Baldor 1/3 hp AC motor ($217), purchased a 10 qt aluminum sauce pot ($12) and aluminum welding ($12) the two aluminum tubes to the bottom of the pot. Absolutecentrifuge also offers the bowl/motor ($550?). Check out other CF companies for what they offer.  Absoultecentrifuge sent their CF'd oil to a reputable testing lab and published the results on their web site. The spec sheet has a long detailed list that includes the water content in ppm, and particle size, etc.  Worth a close look.  The bottom line should be the ppm of water - the lower the better.  Prices have come down from when I purchased the bowl and motor, but the cost was well worth it.  I handle the WVO only TWICE - pour the WVO from the cubbie into a 55 gal drum, pump into my '80 MB 300D from a 275 gal storage tote.
To me the advantages of an open bowl CF are:
(1) low water ppm - my RAW oil 750 ppm - CF'd 1/2 gal/min @140F resulted in 75 ppm - used the Sandy Brea testing system
(2) bowl clean out takes less than one minute - no unbolting - lift the lid and use a kitchen spatula to scoop out the junk 
(3) gravity feed to CF - no high pressure hoses to worry about
(4) ease of operation - walk away and come back when 55 gal poly drum is empty - dregs drain off into a separate jug when CF is shut off for cleaning
(5) don't need to settle - strain through kitchen sieve - start cleaning
(6) process with heat (heated two tank system) or cold (for one tank systems - takes out the FATS - longer VO filter life) 
I know this will create a vigorous debate, but there have been many posts about the effectiveness of the DC unit dealing with the "removal of water".   Worth a look before a purchase is made.  

Mikeoil - 
Answers to your questions:
How often, on average, would a centrifuge have to be cleaned?   
It depends on how dirty is the WVO.  Some oil has more fine particles in it which will require the bowl to be cleaned more ofter.  For example, I get one restaurant's WVO and it is very dark in color (fine flour particles(?) and will require two cycles to clean the oil to a "golden color".  I clean out the bowl after each 15 gals of going through the CF.  The flow rate will be a 1/2 gal. / minute @ 140F ( 1 hr and 20 min).  I process a 40 gal. batch at a time.  The first cycle removes the majority of junk.  The bowl will have two cleaning each about one minute in length with less junk in the bowl with the second cleaning.  The WVO that I collect  from another restaurants has "normal WVO" and only one pass used to clean the oil.  I have run many tests to see how much junk is taken out with each pass and have found the "dirtiest" oil will take two cycles to clean.  A third pass resulted in almost nothing to clean out to be bothered with. The car's heated VO filter has not been replaced in over a year of driving my daily driver.
How much would it cost to purchase it
Cost is a factor no matter which system is chosen.  After reading the CF posts on the WVO forums, reading how well or not the de-watering capabilities (my bottom line), I chose the OPEN BOWL centrifuge system.  I also chose to purchase the bowl and motor ($750 in 2005).  As I said in the above post, prices have come down ($400 or so for bowl and motor).  
 
I am in the process of assembling an OPEN BOWL centrifuge system for a truly mobile system to be mounted in the bed of my '69 Chevy 3/4 ton PU.  I installed a Perkins 6 cylinder/354 ci diesel coupled to a Fuller 10 speed manual tranny.  The Baldor 1/3 AC electric motor will be powered by a 2000/4000 inverter;  a 10 qt pressure cooker will house the Simplecentrifuge bowl;  one Shureflo pump will circulate the WVO and CF'd oil to two 15 gal poly (one for WVO and one for CF'd oil.  A large FPHE will use coolant water will heat the oil as it gravity feeds into the CF.  It will be a duplicate, with some obvious component substitutions, as I have in my garage to process the WVO.  

**I have nothing to do with WVO Designs at all.  Just a very satisfied customer**
 
I picked mine up from WVO designs.  Unparalelled customer service and the 'fuge works great.  I clean mine out about every 50 gallons or so.  Takes 4-5 minutes.  I have a plastic paint scraper that I cut to the same with as the bowl so one pass and it's clean.  
My 'fuge was about $1,000 with a water heater element in a purpose-build housing.  Seriously, these folks are great.

I process 30 gallons of Veg oil a day. Mostly because I use two 30 gallon water heaters as my storage  and heating tanks. I clean out the bowl (I have an Absolute Centrifudge) every 60 gallons.
My system works with simple timers. I heat the oil first in one of the old water heaters mounted above the Centrifuge. Usually about 6-8 hours.  Then it drips into the bowl and empties into a 30 gallon bucket under it for 4 hours. From there a pump pumps the oil back into the second waterheater that is not heated. Then it drips into the 'fuge bowl again and goes into the bucket again then the timer turns the pump back on and it starts over again until I flip the lever and it goes into a ready to go bucket also 30 gallons. I usually run it in the machine 4 times one of those is heated before it goes in.
I can hang a 1 micron filter bag under the centrifuge and it drips right though it for 4 hours. I came up with the 4 passes because I  cleaned the bowl inbetween each pass to see how much was in it. It went down every time and after the fourth pass there was nothing in the bowl. It costs me about $1 per day in electricity to filter 30 gallons. I used a Kill-a-watts meter to record the process. I round up and tell people it costs me 4 cents per gallon to filter.
I spent $1000 on the ready to go model a few years back. I am pretty sure buying filter bags and heating the oil would be more expensive then my fuge.