Hmmmm.... I forgot that the cranks are usually forged. I'm not sure how welding to forged pieces works, especially as forged materials are usually much stronger than cast or CNC-ed pieces which probably would not be a problem welding too. Perhaps welding two cranks together could work as suggested below? I think I'll check in with either a materials and/or a welding forum to see what people's opinions are on welding forged pieces together. I guess I could always purchase some KCNC cranks as they are not forged.
As far as the cranks themselves go, I think KNEEL brought up a good point about the precision needed in the pedal threading angle. As far as CNC-ing cranks, that was my original idea that I had initially discussed with my uncle and he had suggested welding instead as a cheaper, better, and less time consuming option. I have CAD skills (I'm an architect) as well as the specs and diagrams for both ISIS and BB30 interfaces so I could do the design work, but I would have to spend a while drafting and designing something that I have no real idea will actually come out properly. It would be a bummer to spend hours designing something, then hundreds of dollars getting it CNCed just to find out something is off and I have to start over. I was thinking I could bring it to a welder and have them do the work. If they don't do it properly, I could just chop the shoddy work off and start over (with a new welder!)
I didn't mean 'cheap' cranks, but rather 'inexpensive' nice cranks found used on eBay. I was thinking a BB30 set of the aluminum Cannondale SI cranks for my race bike as they are quite nice, light, and have a good profile to them that would be conducive to welding a new piece on there (it's oval shaped like a velodrome - it doesn't have the slight indentation on the insides of the crankarms like most cranks). For my other bikes, any cranks would do, Deore/LX level or Sora/Tiagra would be fine.
I'm not familiar with BMX stuff, so I'm not sure whether I could fit different spiders on those tubular steel cranks to fit my different needs (or what type of bottom brackets they use and if they would fit in my BB shells). For my touring, mountain, and even track bike, I wouldn't be too concerned about weight but I'd rather not have a huge beastly crank on my racing bike. I'll look into it...
I'll also get back with info and opinions from a welding/materials forum...