Would it be worth buying a Mac Pro (higher end model) and using that to mine BitCoin? If so then what should I expect in return as far as mining goes?
@Lohoris did not stress it enough. Do not invest on CPU/GPU hardware with the intent of mining Bitcoin. What you'll spend on electricity will be several times more than the return in Bitcoin (yes, that's how little BTC you will make).
Even low-end ASICs (which are several orders of magnitude more Hash/Watt efficient than CPU/GPU miners) are barely breaking even, even after the price reached 1000USD+. I know because I have one!
If you're asking if it's worth buying one just for mining? Absolutely not. For mining purposes there's nothing special about the MP.
When it comes to mining the simplest way to express how fast you can mine is that ASICs are faster than GPUs which are faster than CPUs. There is of course variation within those categories, but in general that's what you're looking at.
Now, when you are trying to figure your return on investment if your question was "I have to buy a mac pro for some other purpose, would it be worth mining on it?" then maybe, assuming there's some software supporting the GPU set in it. Then all you'd have to do is compare the difference between the wattage it consumes while you're mining with what your return is.
If you were asking because you have $3000 burning a hole in your pocket and you're thinking that the MP will still be useful if bitcoin valuation plummets or someone releases some low-cost mining hardware out of nowhere - no, still probably not. Because you could simply pick up some lower cost ASIC hardware that would do just as well as the MP for $300 and then still have the opportunity cost of that other $2700.