This "delrin is 4 times the cost of aluminum" keeps coming up, but as I've said in other threads, I'm not sure where you guys are pricing your delrin. Here's an example using a place that even the average person can buy from:
Using grainger.com, let's price a 5/8in x 3in x 6in block (should be enough to make one of any of these cases) in delrin and aluminum.
Delrin:
http://www.grainger.com/Grainger/DELRIN-Sheet-Stock-2XMF2?Pid=search
This is a 5/8in thick piece of white delrin (Atomic offers this color) that's 24in by 48in. By my math, that should make 64 3in x 6in blocks at 5/8in thickness. The cost of that sheet is $382.75, so per 5/8 x 3 x 6in block it comes to $5.99/block.
Aluminum:
http://www.grainger.com/Grainger/Flat-Stock-2EZT4?Pid=search
This is a 5/8in thick bar of aluminum (same thickness as above) that's 3in wide (same width we cut the above delrin to) and 6ft long (72 inches) long. That should give us 12 6in x 3in pieces when cut. The cost of that bar is $93.40, so the cost per 6 x 3 x 5/8 inch bar is $7.78/block.
So using the same sized block (5/8 x 3 x 6 inch), which material costs more? And again, this is a place anyone can buy from, and larger purchasers can get it at a bulk rate from other much cheaper places. This example is using white delrin, but black delrin is only a few dollars per sheet higher (Grainger isn't listing black delrin or I'd have used that). And white delrin is "Delrin 150", whereas black delrin is typically sold as "Delrin 100", which is a little lower in impact resistance than natural (white) delrin.
The point being, that I also expressed in another thread, is that delrin isn't "3-4 times more expensive" than aluminum.
* and to those machinists out there, yes, I know that you can't get these exact numbers of 3in x 6in blocks out of the material sizes I listed above, as you'll lose about 0.030 -0.060" per cut, but the idea is the same nonetheless.