I wish the market were as generous as your prediction. You're lucky to get 50% for your machine in 2 years.
Since they moved to Intel or the downturn of the economy, don't expect people to pay top-dollars for your electronics; Apple or otherwise.
So realistically what you can expect to get is probably at a rate of 25% depreciation after a year andthat is already generous!
Even that would be very generous.
I've sold a few Macs in the last couple of years, and the best I could get (with AppleCare and other bonus add-ons) was 50% of it's original sale price after 6 months of age.
The problem, is that when Apple updates the product line, the maximum value shifts.
People look at what they can get the new machine for with all these standard features that you had to buy as options for yours.
So, options you paid for, are frequently standard in the new configuration. And, the new configuration is usually the same price or less. So, then you've got your machine being compared against a new one which has all your features, faster processors, a full new warranty, and is NEW.
So, when people start doing deductions, they come in with pretty low offers. I've played the game and lost every time.
The only times I've felt that I've come out O.K. are when I've kept a Mac for multiple years. Then, selling it at a significant loss isn't as hard because I've gotten a lot of use out of it.
But, selling a nearly new machine, just isn't worth it. Don't ever buy a computer with selling it in mind. If you are already planning to sell it, then it isn't enough computer for you (or isn't the right fit for you).
I purchased this Mac Pro with a timeline of 4 to 5 years of use expected. If I were to consider using it for any lesser amount of time, I wouldn't have purchased it at all.
And, beyond that 4 to 5 years, I expect I will still keep it. It will just get handed down through the family.
If you are already worried about resale value, you are not buying the right machine.
From my experience, the last two Macs I purchased and sold cost me $2300 (with a corporate discount) and $1600 (with a corporate discount).
The $2300 system was about 5 months old when I sold it. Apple had just released a new revision. And, the absolute best I could get for it was $1100.
The $1600 system was about 3 months old when I sold it. And, the best I could get for it was about $900.
Both systems had AppleCare coverage for 3 years.
And, these were sales that took place in the last 3 years.
The problem, is that Apple will release a new revision. That new revision will come with the stuff you paid extra for. So, the value of those extras will be depreciated. The value of the machine will be depreciated since it is no longer the current model. And, it's price will partly be set by:
1) The price of the new machine with all it's standard options
2) The price of a refurbished machine similar to yours
3) The fact that yours is not option 1 or 2 and will therefore receive offers lesser than both.
If you are going to spend the money, get something that will serve you for a long time.
Or, if you are looking short term, buy the cheapest model that will serve you for that amount of time.