The differences between 20k and 40k pulsations/min doesn't seem to be a lot.  I posted before a few NCBI articles that done research on them (scientific randomized controlled trials). 
That said, nowadays the price difference between Pro 600/1000 vs Pro 2000 and above isn't much as well....so people may still be more comfy in getting the 2500 etc.    I used to get my Pro 600 for just $25 shipped from Amazon UK, really brainer.   Item cost was 12.50 pounds and shipping was just additional 1 pound+ when tagging to an existing order.
You pop 2pcs into the mouth (remember not to press too hard to prevent enamel erosion) still cheaper than the ebay 2500.    17600 oscillations/min and 40000 pulsations/min anyone?  
But that deal is now gone.  Wanted to buy a couple more.....
Kids shutting between 2 places, so need twice the number anyway....
Pro 600/650/690 is 8800 and 20k.  
Pro 500, 450 is 7600 and 20k.
	
	
	
		
		
		
		
	
	
Well, ok....now it's 16.64 pounds (after minusing the VAT when you checkout, checkout page can see.....  my above 12.50 pounds figure is after deducting the VAT for international orders.  (17.5%?)  
Amazon lists it as 19.97 now.  
So not too bad.   The last time i saw it when i wanted to re-order, it was listed in the 20+ pounds range. Bo hua.
Oral-B Pro 600 White and Clean Electric Rechargeable Toothbrush Powered by Braun, Ships with a UK 2 pin plug
£16.64 - Quantity: 3
Sold by: Amazon EU S.a.r.L.