To be honest, what I think Apple can't make a cheaper device without substantially degrading the design
- the iPhone's design incorporates glass or metal, this is to wick away heat, so these can't really be compromised without making the device larger
- All current phones being sold have a Retina display, hence removing it would break compatibility with software that presumes it.
- Likewise, removing or downgrading the parts would sacrifice build quality or features
Notice how the previous (now cheaper) models are only available in the smallest capacity?
With the Mac, the only way a cheaper mac is being built is either Intel capitulating on CPU prices, or Apple using AMD A-series CPU/GPU's. The parts aren't bad, but because of AMD creating parts that suck back more power, Apple probably won't ever consider it.
So I don't think we'll see a cheaper "iMac" but it's certainly possible to make a cheaper MacMini, because all things considered, the hardware hacking community has done it repeatedly. It's not a question of how, but why. The reason the Macmini costs as much as it does is because all the parts fit together well. Try doing that with a handful of parts you buy separately. Most of what you see as "macmini clones" out there are using Atom, Celeron, or AMD E-series APU parts, not i5's and i7's. And the reason is power requirements. If Apple want's to lower the barrier into getting into the Mac OS X ecosystem, all they have to do is licence a community-supported version for Whitebox/OEM PC's (any non-apple parts do not come with drivers.) I don't see that happening anytime soon, if ever given what happened with the Mac Clones. Given how much people hate Windows 8 right now, this would be a way to provide an alternative.