Dirt cheap 3-12v DC/DC isolatior chip?
For isolated data and power on a single chip, see the Analog Devices isoPower iCoupler parts range.

These ICs provide "Signal and Power Transfer Across Isolation 
Barrier Using Microtransformers" - in other words, there are integrated solid-state transformers on the die of the IC, as well as oscillator-rectifier-filtering to drive power outputs with 2.5 kV isolation.



Dirt-cheap? Not really. 

However, if you have an interesting idea, Analog Devices has been known to not only support designs with free samples, they also throw in their engineering expertise, with design support engineers jumping in to help you work through any kinks in your requirement. 



See also this answer for further insight into the iCoupler product line.

I am not associated with Analog Devices in any way.