I'd like to bring my own experiences with building a "Mac Pro Lite" to the table, if I may. Feel free to skip past the backstory, it's long and kinda boring.
[Backstory]
I was very unhappy about the Apple situation. I refused to go Apple because they made nothing for me - the pro machine was way too much, I didn't want the iMac for various reasons outlined here, and I had delusions of incompatibility with OSX. I had always loved the mini, but it just didn't have enough oomph for me.
My father constantly complained about his old Compaq. It was a PIII w/ 256 ram, integrated graphics. He had lost the hard disk a number of times and it just wasn't performing for him. At this stage I harboured a secret desire for Apple products but I had neither the money nor the courage to buy my own. I rather cunningly pushed him towards getting a 1.5Ghz G4 Mini - we went to the local Apple retailer, he got his educational discount (he's a part time lecturer) and was very happy with his purchase.
I helped him set it up and he instantly commented on how much better the video display was - this had nothing to do with graphical bling, the actual hardware was far superior to the old computer, which left the screen streaked and washed out. He spent a while with his Mini and started to feel the pinch of the integrated graphics while manipulating large images in iPhoto, Photoshop and Osyrix (an X-Ray manipulation program he uses for work). I suggested he have a look at the (then) new Intel iMacs because they had superior graphics. We picked up a 2.0 GHz 17" iMac, and he's absolutely loved it.
About 6 months after he got his Mini a friend of mine asked me if I wanted her old iMac which "wouldn't work." I had no idea what to expect, but I came to possess a 400Mhz G3 slot loading (DV) with 128mb ram. It was easy to fix, but I couldn't really do much with 9.2 on it.
My grandmother had seen iMacs on display in the store, and when I explained to her that the whole computer was in the screen she immediately wanted one. Her Dell was cluttering up the desk like no-bodies business, so had to go. I passed it on down to my nephew after doing a few upgrades and it suits him fine. My grandmother got her shiney new 17" with the GMA950 (I used my educational discount), and I instantly installed windows XP via bootcamp for her. She uses it to manage her photos, her emails and browse the web. I'm slowly trying to wheen her off windows and on to OSX.
As a reward for being nice and setting up the computer for her, she agreed to buy me OSX 10.4.6 when we ordered her iMac (we bought it online). I greedily installed the firmware update on the G3 iMac, popped in another 256mb, swapped the hard disk for a 120gb model and installed Tiger. I was blown away! After I'd ditched all the Apple-supplied programs and installed all of my own (Firefox, VLC, Adium etc) as well as Microsoft Office for Mac, all the misgivings I had about the operating system vanished. It was FAST. It was compatible enough that I didn't have to screw around too much to integrate it into my windows-based home network. It was QUITE! I instantly fell in love, and the little G3 that could became my main computer.
Over the next few months I began to notice little things that niggled at me. Some were integral to OSX (it's such a pain to automatically mount network drives!) but a lot were due to the poor old thing's age. If the mac was busy, I could type a whole sentence before it appeared in Word. The USB 1.1 was a pain - and only 2 ports. Clearly I needed to upgrade the hardware.
If I bought an Intel Mini, I'd have a lackluster video card, laptop components and very limited expandibility. If I bought a Mac Pro it would be this behemoth sitting on or under my desk, costing me far more then I could afford. The iMac was attractive, but it still used a laptop CPU on a slower bus with more expensive memory, and would be a pain to get in to upgrade anything. I really wanted a Intel Cube to come along. To that end, I started looking around for REAL Intel Cubes. I found a few sites that put PC components inside a Cube, and a few more that put OSX on to PC's. Looking deeper into the PC components I would need I was able to spec out I found a beautiful case and I decided to forego modding a Cube (which I felt was a little disrespectful, anyway).
The parts I chose were:
CPU: Intel e6700 (2.66Ghz, 1033Mhz FSB)
RAM: 2x 1GB (800Mhz)
Motherboard:
Commell LV-674E
Video: NVidia 7950GT (a nice version from EVGA)
Case:
Hoojum Cubit 3 in "Pure White"
Hard Disk: 500GB SATA w/ 8 MB cache
Optical Disk: CD-(everything)/DVD-(everything) reader/burner
These parts and various others to make it all work together cost me nealy AUD3.5k - so much for cheap! Luckily I bought things over time. I added a 23" ACD, wireless keyboard and mouse, etc to really complete the "Apple" feel.
Putting OSX on it was a chore, but I insisted (to myself) that I do it the "proper" way and not just download some image of the internet. I went through with dumping the image from my purchased 10.4.6 disks, modifying them manually, rebuilding the image and booting that. I feel that although I violated the apple EULA I did it in the spirit of fair use and inter-operability. In the end I needed to hunt down sound driver kexts and some video kexts, but I was incredibly happy with my brand new OSX machine. It was everything I expected it to be. However, I still fight with it on a weekly basis due to heat and compatibility and noise issues.
[/Backstory]
The bottom line is that the specialised components needed to make my very own Mac ended up costing far more then I expected. The special motherboard was nearly AUD500, the PSU was close to 300 and the case was as well. I would have saved more and ended up with a more powerful system had I bought a Mac Pro.
Just because you can buy PC components for $800 and give yourself Mac Pro performance, you will not have the equivilent of a "Mac Pro Lite." Even with OSX it's still just another PC. I don't see how a new machine would make sense to Apple. They would have to invest in designing a completely new motherboard architecture, new case, have to deal with support of the new line. YOU may be able to design and build a low budget killer rig, Apple won't. Coupled with their past failures in this arena and Job's desire for a completely closed system, I see absolutely no reason for Apple to enter this market.