MOTM

The vocoder thread has been covered before, but it may have been 
several years ago.

To build a vocoder with 20 bands for instance requires 40 bandpass 
filters (to do this right actually requires that each of the bandpass 
filters is actually 4 filters, this is required to get flat response 
across the band), 20 envelope followers and 20 VCAs.  There is usually 
a noise source to simulate sibilance as well.  

This is a lot of ciruitry and would be very expensive.

How many on this list would pay $1000-$2000 for a Paul S. Vocoder?

Of course there are cheaper ones out there, but SynthTech is about 
quality.  There are also many software vocoders which work very well.