Only if you do not want the same level of performance a Xeon workstation/server class CPU, error checking & correcting (ECC) RAM (good for long calculations), PCIe based flash storage and dual workstation OpenCL GPUs offer together in a good package.
But then again, most people spouting that nonsense about building a faster or better computer for less than a Mac Pro do not consider those aspects, as they only look at one or two components at once and ignore the rest.
Are you in Apple marketing?
I just bought a Dell Precision Workstation T3610 for $2244.73. Ordered 27 March, to be delivered tomorrow 7 April.
Same 6 core "Xeon workstation/server class CPU, error checking & correcting (ECC) RAM (good for long calculations)" E5-1650v2 3.5 GHz CPU and 1866 RAM as the $3999 new Mini Pro.
Has a 1 GiB Quadro GPU - I don't need dual GPUs, and the GPGPU stuff that I do use is CUDA-based. Don't inflate the price with (for me) useless mid-range ATI GPUs.
Has 8 GiB of 1866 ECC RDIMM (2*4GiB) and 8 DIMM slots. No amount of money will get a new Mini Pro with 8 slots. Ordered another 72 GiB from Newegg (4*16GiB + 2*4GiB) from Newegg, so will start with 80 GiB. Will upgrade to a supported 128 GiB config when needed. Apple doesn't support 128 GiB.
Has 685watt power supply. What's with Apple and the 20th century 400watt PS in the new Mini Pro?
Has two PCIe 3.0 x16 slots (one used by the Quadro), one PCIe 3.0 x8 (physical x16) slot, one PCIe 2.0 x4 slot, one PCIe 2.0 x1 slot and a PCI slot. No amount of money will get a new Mini Pro with PCI slots.
Has a DVD-RW and BD-RE drive. No amount of money will get a new Mini Pro with optical drives (from Apple).
Also let your friend take a look at HP and Dell and Lenovo workstation computers of similar kind, they are often more expensive and have a much bigger energy consumption.
The "much bigger energy consumption" is pure BS FUD. The others use the same CPUs and chipsets as Apple, and will use the same power as the Dells, HPs and Lenovos. (My office T1650 E3v2 usually sits at less than 40 watts for the system box.)
You need to "put up or shut up" on this claim about power consumption.