Ordered a new 13" Macbook Pro Retina - cost around $2800.
Just sold my mid-2011 11" i5 1.6Ghz 2Gb RAM 64Gb storage Macbook Air for $550.
Cost me $1250 so I reckon that's pretty good resale for a 3 yo machine.
By the time I claim the GST back and credit myself the $550 the new one will only have cost $1900. Excellent!
I've sold 3 Macs and never got less than 35% resale so far at 3 years of age - its nice to get something back worth having.