Forgive the length of this reply, I'm not the best at putting my thoughts in to words. I hope, however you appreciate my logic.
The figures I quote are taken from a quick ebay search so may not be entirely accurate but should be somewhere close to what to expect.
On ebay UK today, I searched for a 2010 Macbook Pro 15 and found that the majority were selling for between £700 and £900. meaning they have depreciated about 50% over 3 years (£800ish) depending on the model and specs of course. I then searched for a 2009 Macbook pro and saw them bidding for anywhere between £200 and £400 meaning the had dropped around 75% over 4 years (£1100+). To me, the depreciation is the real cost of an item. If you buy a computer for £1500 and sell it 3 years later for £750, it cost you £250 a year. If you buy one for £1500 and sell it 4 years later for £350 Its cost you almost £300 a year, either way, less than £1 a day. So whether you keep it for 2 years, 3 years, 4 years or even 5 years the depreciation will only increase as time goes by (apparently about £250-£300 per year, but obviously it will depreciate more the first year and progressively less for year of ownership).
So, buy yourself a rMBP and then immediately start dropping a £1 coin in to one of those giant novelty Whisky bottles every day and each year you'll have saved £365 (£366 in a leap year!) towards the cost of your updated replacement. £1 a day should, by my calculations be far more than the depreciation of the machine so you may even find yourself in a position to buy a higher spec, brand new rMBP after 3 years by combining you Whisky bottle with the resale price of the old one.
A final point, if you believe that £1 a day is too much to be spending on a computer then a Macbook Pro probably isn't right for you and you might be better off going for a cheaper Mac or even a PC. I use computers for hours everyday so would happily pay £1 for that privilege.
