Ask HN: cheap way to control voltage wirelessly

There are a lot of knobs that can be turned by controling the voltage on an input in real time. In my field, people usually who want to do this just buy whatever National Instruments is selling for $1500. It's expensive and involves a mess of wires, but it works.

Does anyone here know of a cheap way or two to wirelessly control a voltage (say from a netbook or phone)? Cheap here means in the $1-$25 range.